https://orcid.org/0000-0002-6890-2823
Abstract
Background
Postural stability is essential for functional independence in the pregnant population. The contradictions between existing studies and the lack of consistent characteristics in the strategies used by pregnant women for postural control demonstrate the need for further investigation.
Objectives
The aim was to review the available literature on postural strategies throughout pregnancy in both static and dynamic conditions and to provide an assessment of the quality of these studies in terms of methodological issues to identify the reasons for the inconsistencies in findings between research centers.
Methods
Literature searches were conducted using PubMed and EBSCOhost Research Databases. The latest search was performed on September 01, 2024. The review was restricted to longitudinal, cross-sectional, case-control, and descriptive studies focused on the effect of pregnancy on the stability of future mothers, with the following criteria: healthy pregnant women and singleton pregnancies. Trials were excluded if they were restricted to multiple pregnancies or considered various kinds of interventions. The methodological quality was evaluated using the criteria proposed by Downs and Black. Data items such as information on study design, characteristics of the study sample, equipment used, stability task performance, and outcome measures were presented.
Results
The final analysis comprised 22 articles, including a total of 641 pregnant and 296 nonpregnant women. Research results in both static and dynamic conditions are inconclusive, showing either a decrease, no change, or improvement in postural equilibrium as pregnancy advances. Importantly, the results indicate that women in advanced pregnancy may be at increased risk of falling when their vision is compromised.
Discussion
A lack of homogeneity in the study groups and a small number of longitudinal analyses were observed. The methodologies applied and the postural indices used to measure body sway varied across the studies. Our findings can serve as basic data for health promotion programs to encourage safe daily activities in pregnant women.
Figures
Citation: Forczek-Karkosz W, Masłoń A (2024) Postural control patterns in gravid women—A systematic review. PLoS ONE 19(12): e0312868. https://doi.org/10.1371/journal.pone.0312868
Editor: Jean L. McCrory, West Virginia University, UNITED STATES OF AMERICA
Received: October 27, 2023; Accepted: October 14, 2024; Published: December 27, 2024
Copyright: © 2024 Forczek-Karkosz, Masłoń. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Data Availability: All relevant data are within the manuscript.
Funding: The author(s) received no specific funding for this work.
Competing interests: The authors have declared that no competing interests exist.
1. Introduction
Postural control is a fundamental ability that not only provides the basis for standing and walking independently but also facilitates the performance of manual tasks. An erect bipedal position is regulated by sensory inputs (visual, vestibular, and somatosensory) to maintain postural equilibrium and proper alignment of body segments with respect to gravity [1–3]. Joint configuration, the center of mass (COM) position, and balanced muscles all contribute to optimal postural alignment. Understanding the motion of the COM with respect to the base of support (BOS) offers insights into balance control strategies [4, 5]. In static posturography, COM oscillations are represented by the center of foot pressure (COP) displacements [6]; thus, these measures are mostly used for postural balance assessment. However, increasingly more scientific reports emphasize that the goals of postural stability are broader than maintaining the COM above the base of support [7].
Stability is often described as being static or dynamic. Static stability is defined as the ability to minimize movement of the center of gravity within the base of support under a given condition [8]. Defining dynamic postural stability is more challenging. It is the ability to transfer the vertical projection of the center of gravity around the supporting base [9]. It has been measured following a perturbation of the support surface [10], a perturbation of the individual [11], or by requesting the individual to maintain balance following a change in position or location [12, 13]. There are two main processes used to restore or maintain postural stability: anticipatory postural adjustments (APAs) and compensatory postural adjustments (CPAs). APAs occur prior to movements and are a feedforward process that counteracts an expected perturbation [14]. They are considered the first line of defense for postural stability in anticipation of perturbations and have been observed while sitting, standing, or walking [15]. In contrast, CPAs are responses to external postural perturbations and are therefore under the control of feedback mechanisms [14].
Scientists point out that in everyday life, postural stability must be controlled in such a way as to ensure both the control of posture and the completion of various tasks (e.g., reading, talking, lifting an object). Research shows that performing more demanding tasks (e.g., reading) may result in a greater reduction in sway magnitude than accomplishing less demanding tasks (e.g., looking at a blank target) [16–18]. Given the complexity of maintaining postural stability, assessments should not only measure the magnitude of sway but also the ability to modulate it depending on the task, environmental, or individually variable factors.
One factor that may undoubtedly influence the need for adaptation to maintain balance and optimize joint load distribution is pregnancy. Typical pregnancy-related adaptations include a profound increase in body mass, primarily in the breasts and abdomen, fluid retention, and connective tissue laxity caused by hormonal changes [19–21]. These adaptations are followed by an increased anterior pelvic tilt [22, 23], increased lumbar lordosis [24, 25], and spine extension [26] to prevent the COM from shifting. The increased lumbar lordosis helps maintain an unchanged anterior-posterior position of the COM as pregnancy progresses [26]. Lateral stability, however, may be preserved during pregnancy due to an adaptive increase in stance width [23, 27].
The analysis and interpretation of postural sway should be carried out carefully because the data processing technique may affect the structure of COP variability [28]. Some authors emphasize deterioration in postural stability in the pregnant population [e.g., 29, 30]. Furthermore, some identified pregnant women as more prone to falls [31], which significantly increases adverse outcomes for mothers [21]. However, it is also proven that stability during pregnancy remains unchanged [32]. The aforementioned contradictions between existing studies and the lack of consistent characteristics in the strategies used by pregnant women for postural control demonstrate the need for further investigation into postural adaptations during pregnancy in terms of stability issues. It is of great importance to make women more aware of the alterations responsible for many discomforts in different body positions and during various activities.
1.1. Objectives
The aim of this paper is twofold: first, to review the available literature on postural strategies throughout pregnancy in both static and dynamic conditions; and second, to assess the quality of the studies in terms of methodological issues to identify the reasons for the inconsistencies in research findings on postural control in pregnant women.
2. Methods
This systematic review was conducted according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA 2020) guidelines [33]. A review protocol was not prepared.
2.1. Eligibility criteria, information sources, and search strategy
The inclusion criteria established by the authors were peer-reviewed papers published between 1990 and 2023 in English. The review was restricted to longitudinal, cross-sectional, case-control, and descriptive studies. Eligible studies should investigate the effect of pregnancy on the postural stability of future mothers with the following criteria: healthy pregnant women and singleton pregnancies. Trials were excluded if they met any of the following criteria: restricted to multiple pregnancies or considered various kinds of interventions (see Fig 1). Systematic reviews and meta-analyses were only included as background. Conference proceedings, letters, commentaries, editorials, abstracts only, or case studies were also excluded.
The main literature searches were conducted using PubMed and EBSCOhost Research Databases: MEDLINE, SPORTDiscus with Full Text, Rehabilitation & Sports Medicine Source, Health Source—Consumer Edition and Health Source: Nursing/Academic Edition. These databases were selected because of their broad inclusion of multidisciplinary topics within the Biomedical and Health Sciences domain. Each database was searched including the same range of years. The search strategy clustered terms used to describe studies investigating the effect of pregnancy on the postural stability of pregnant women (see S1–S3 Tables). The following search terms were used: “postural control” OR “postural balance” OR “balance” OR “postural stability” OR “body balance” AND (“pregnancy” OR “pregnant”). The restrictions applied to date and language: only English language studies published within 1990-2023were included. The latest search was performed on September 01, 2024.
2.2. Selection and collection of data
Two independent reviewers (W.F.K. and A.M.) analyzed the retrieved articles, taking into account the eligibility criteria. Initially, the papers were screened by checking the titles. Duplications were identified within the papers found, which included both duplicates between the databases and internal duplicates of the same research being published in more than one format. After removing duplicate publications each of the two reviewers read the abstracts of all articles, selected relevant articles according to the inclusion and exclusion criteria, and defined a list of articles for full-text reading. In case of disagreement, consensus on which articles to screen full-text was reached by discussion. Then, the researchers independently screened full-text articles for inclusion. In case of disagreement, consensus was reached on article inclusion or exclusion by discussion. Each reviewer independently reviewed each article to search for the data items. Data were extracted from article texts, tables, and figures.Some of the important information were copied and pasted into tables (e.g. technical details of the equipments used during the experiments), but the results were carefully selected and placed in tables in summary version.Afterwards, the extracted data were compared, and for any disparities, both reviewers determined the best-suited set of data through discussion.
2.3. Data items
For this systematic review, the following data items were presented: authors, title of the paper, year of publication, study design (Table 2). Other data items concerned subjects’ characteristics such as age, gestational week, parity, and sample size of the study (Table 3), as well as the objective of the study, outcome measures, the experimental setup regarding equipment used and tests performed, and the results of the study (Table 5).
2.4. Study quality assessment and synthesis methods
Due to key differences in the comparisons performed in each study and various outcome measures, we could not perform a meta-analysis of the included studies. Instead, we narratively synthesized the evidence.
The study quality was assessed using a modified checklist by Downs and Black [34] (Table 1). Because the included studies did not concern interventions, all questions in the Downs and Black quality assessment instrument that referred to interventions were omitted (14 questions were removed). Thus each article could score up to 13 points. Two reviewers assessed each study independently, and then assessment scores were compared and discussed in cases of disagreements until consensus was made. Table 2 provides the scores for all studies included in this systematic review. Each article could score up to 13 points. The authors recognized an article as of sufficient quality if it scored at least 6 points (representing a value of 50% + 1 point).
3. Results
3.1. Study selection
After screening the titles and abstracts, from a total 243records from EBSCOhost, only 35 remained for the analysis, while from a total 56 PubMed records, only 7 remained for the analysis. Additionally, through other sources, we identified 8 papers. After full-text assessment, 22 studies were included in this systematic review. The process of study selection in this review is described in the PRISMA flow diagram (Fig 2), along with the reasons for exclusion.
3.2. Study characteristics
3.2.1. Study design.
The studies are presented in chronological order based on the publication year (Table 2). Ten of the studies were longitudinal, seven were cross-sectional, two were descriptive, two were retrospective, and one was a case-control study. Among these qualified articles, 14 focused on static stability, four on dynamic stability, and the remaining considered both. Considering the countries where the studies were conducted, seven were carried out in the USA [27, 29, 30, 38, 42, 43, 49], four in Brazil [32, 37, 40, 52], and three in Japan [35, 45, 46]. Other countries, like India [39, 51], Turkey [41, 44], and Poland [48, 50], each had two studies on this issue. Finally, investigators from the Republic of Korea [36] and Egypt [47] provided one paper each.
3.2.2. Group characteristics.
Among the analyzed studies, 15 (out of 22) included control groups (Table 3). The mean number of participants for the pregnant group was 28.8 ± 20.7, and for the nonpregnant control group, it was 21.1 ± 12.3. The mean age of the pregnant women included in the analyzed studies was 28.8 years, whereas the mean age of the control group was 26.9 years.
In 12 out of 15 studies with a control group, one or more of the following factors were considered in matching the research and control groups: age [27, 29, 35–39], body mass [29, 35, 40, 41], BMI [29, 30, 35, 40–43], and body height [29, 35, 36, 40]. Considering age and body height, no significant differences between control and study groups were observed. However, in four studies, control and study groups varied significantly in body mass and BMI. In two of them, this was more understandable since it was observed for women in late pregnancy [35, 41], while in the remaining two, significant differences in either body mass [40] or BMI [38] were observed even for women in early pregnancy. In 14 studies, information about parity was included. Among those including nonpregnant control groups, some provided information about parity for both pregnant and nonpregnant groups [29, 41, 43], while others provided it either for the nonpregnant [38, 40] or pregnant [27, 37] group. One study included figures on both parity and gravidity and showed that the groups were matched for these factors [41]. All pregnancies of the women included in the analyzed studies were singleton. The longitudinal analysis of postural stability throughout the three trimesters of pregnancy was shown in five studies [27, 29, 32, 40, 44], while eight studies analyzed two trimesters (2nd and 3rd [30, 36, 42, 43, 45–47] or 1st and 3rd [37, 48]). Four studies included women in either the 1st [49, 50] or 3rd [35, 39, 51] trimester of pregnancy. Furthermore, three studies provided an analysis of postural stability across all three trimesters; however, this was not a longitudinal analysis as the group was tested once and consisted of women at different stages of pregnancy [38, 41, 52]. Four of the analyzed studies assessed postural stability postpartum, mainly considering the first six months after delivery. The number of postpartum experimental sessions ranged from one (6–8 weeks postpartum [29]), through two (2 and 6 months postpartum [48, 50]), up to three (6 weeks, 12 weeks, and 6 months postpartum [27]). In most studies, a single data collection session for the nonpregnant control group was planned, whereas in one study, the control group was tested using the same time scheme as the research group [27]. The division into trimesters varied greatly between the studies. Furthermore, in some studies, despite a division into trimesters, there was no information about the week of pregnancy when the data were collected [38, 39, 41, 46, 49].
3.3. Risk of bias in studies
The methodological quality concerning the risk of bias in the included studies was evaluated using the criteria proposed by Downs and Black [34]. These characteristics allowed for comparing articles, especially regarding their methodological design, sample size, measures of exposure, and definitions of outcomes. Because the included studies did not concern interventions, all questions in the Downs and Black quality assessment instrument that referred to interventions were omitted (14 questions were removed). Thirteen out of the possible 27 questions were used for quality assessment, as presented in Table 1.
Using the modified Downs and Black Checklist, the included studies scored between 8 [23, 26, 33, 39] and 13 [18, 30, 36] points for quality, out of a possible 13 points (S4 and S5 Tables). Categorization of total scores obtained allowed us to determine that analyzed studies were of moderate and strong quality (Table 4). All of the studies clearly described their objectives and main outcomes to be measured and used the appropriate statistical tests to assess the results. Several studies did not adequately describe patient characteristics and main outcomes including not providing estimates of the random variability in the data and actual probability values. In the vast majority of studies, recruitment of the study participants, the staff and facilities where the study was conducted as well as characteristic of patients lost to follow-up information have not been described.
3.4. Results of individual studies
The methods used as well as the results of the studies, are presented in Table 5.
3.5. Outcome measures and equipment applied
3.5.1. Static stability.
In the majority of cases, control of posture in a quiet stance was quantified by the center of pressure (COP) changes in the anterior-posterior (AP), medial-lateral (ML), or combined radial (RAD) directions from a single force platform [27, 29, 32, 35, 37, 38, 40, 48–51]. In two studies, two force platforms were used for each lower limb [37, 45]. Takeda [45] assessed stability while performing the functional reach test (FRT) from a static standing posture using two force plates for each lower limb and a 3D motion analysis system. He evaluated the maximum FRT distance [cm], COP anterior displacement at FRT max [cm], GRF at FRT max [N/kg], leg joint moments at FRT max [Nm/m/kg], and leg and trunk angles in the sagittal plane at FRT max [deg]. Then, in Takeda et al. [46], two stabilometers were used to register COG movements in AP and ML directions. In the study by Ribas & Guirro [52], a pressure platform measuring both plantar pressure and postural balance based on center of force (COF) oscillations was applied. Mean COP sway velocity (Vel) was assessed by Moreira et al., Danna-Dos-Santos et al., and Opala-Berdzik et al. [37, 38, 48]. Other measures of balance were provided by Yoo et al. [36], who assessed the weight distribution index (WDI) using four force plates of the Tetrax® system. Yu et al. [49] carried out detrended fluctuation analysis (DFA), including α (an index of long-range autocorrelation in the data), to measure the positional variability of the COP in the AP and ML axes [cm]. Nagai et al. [35] analyzed the path length [cm] and area [cm2] of the body COP sway. Finally, Sancar et al. [44] evaluated static balance with the Biodex-BioSway TM Balance System (BBS). It was performed with the Postural Stability Test, Limits of Stability (LOS), and Modified Clinical Test of Sensory Integration and Balance (mCTSIB). As a result of the Postural Stability test, the Overall Stability Index (OSI), Medial-Lateral Stability Index (MLSI), and Anterior-Posterior Stability Index (APSI) were obtained, indicating the amount of deviation from the AP and ML axes. The patient’s score on this test assesses deviations from the center. Then, from the LOS test, defined as the maximum angle a person’s body can achieve from vertical without losing balance, the "overall" score of the individual was obtained. The mCTSIB test was used to evaluate the standing balance in different situations that the individual may encounter in daily life. As a result of the mCTSIB test, body sway was calculated, and the sway index scores (SI-Sway Index) for Eyes Open Firm Surface, Eyes Closed Firm Surface, Eyes Open Foam Surface, and Eyes Closed Foam Surface were obtained.
3.5.2. Dynamic stability.
As far as dynamic stability measures are concerned, we found different approaches. McCrory et al. [30, 42] and Ersal et al. [43] defined dynamic postural stability by the response to anterior and posterior translation perturbations of different magnitudes. In these studies, the Equitest platform was used to directly register COP metrics. Additionally, Ersal et al. [43] analyzed COG [cm] and Peak COP–COG [cm]. For the theoretical part of the study, Ersal et al. [43] implemented a two-segmented mathematical model to represent the dynamics of the body. The model identified the dominant strategy of stability in the subjects. Stability indices (Overall Stability Index (OA), AP Stability Index (APSI), ML Stability Index (MLSI)) were provided by the Balance platform at level 8 [41] and at levels 7 and 8 [47]. In two studies, gait trials were considered as examples of dynamic stability. Mocellin & Driusso [40] applied two force plates to measure time and the vertical and horizontal components of GRF, while Yoo et al. [36] measured gait velocity [cm/s] and cadence [steps/min] using the GAITRite system. Finally, Shingala et al. [39] applied four canes (<2.5 cm in diameter) to assess postural balance while performing the Four Square Step Test (FSST) to measure the ability to step quickly in different directions.
3.6. Results of syntheses
3.6.1. Static stability.
Most studies focusing on the static stability of pregnant women were conducted using parameters obtained by analyzing the center of pressure (COP) trajectory in both the medial-lateral (ML) and anterior-posterior (AP) directions [53]. Generally, the authors observed either a deterioration of postural stability in the sagittal plane over the course of pregnancy, no changes in COP sway along the ML direction (or even reduced sway in the third trimester), or no differences between groups. Some studies additionally considered the effects of visual condition (eyes open/closed) and support base configuration (wide/narrow) on the COP area. Most authors observed lower COP displacement areas with eyes open and feet apart, and higher values with eyes closed and feet together. Finally, in a small number of studies, COP sway was found to correlate with feelings of instability during standing, trunk flexion flexibility, and anxiety in pregnant and postpartum women [53].
In the latest review on static balance during pregnancy, the general observations were confirmed, noting significantly greater COP (AP) amplitude in the third trimester compared to the first, but no difference in COP (ML) amplitude [53]. Moreover, they concluded that there is a positive and significant correlation between AP COP oscillations and support base in women in the first trimester. Since Goossens et al. [53] did not consider four papers related to static stability included in our review, we would like to briefly mention the results of these studies. Ribas & Guirro [52] observed significantly greater COP (AP) amplitude in late pregnancy compared to early gestation, with no difference in COP (ML) amplitude. Yu et al. [49] assessed the temporal dynamics of sway and found that pregnant women with morning sickness had reduced positional variability of COP but reported greater perceived instability compared to those without morning sickness. This suggests that women with morning sickness may attempt to stabilize their bodies by reducing overall body sway, resulting in better postural stability despite feeling more unstable. Sancar et al. [44] found no significant change in postural stability throughout pregnancy. However, significant changes were detected in the last trimester in terms of sway tested in the Limits of Stability (LOS) and the Modified Clinical Test of Sensory Integration and Balance (mCTSIB). A significant increase was found in LOS in the last trimester compared to the first. According to the mCTSIB, oscillations were higher in the third trimester than in the second. Ramachandra et al. [51] pregnant women demonstrated larger median velocity moments and mean AP sway velocity compared to nonpregnant women across all tested sensory conditions. Although ML sway velocity did not show any statistically significant difference, ANCOVA results suggested a significant difference in mediolateral sway velocity in the eyes-open feet-apart condition on a firm surface and the eyes-closed feet-apart condition on a firm surface between pregnant and nonpregnant women. There was a larger velocity moment and anteroposterior postural sway velocity in pregnant women in their third trimester compared to nonpregnant women when exposed to different sensory conditions.
3.6.2. Dynamic stability.
What distinguishes static stability from dynamic stability is the fact that in the latter case, both the base of support (BOS) and the center of mass (COM) are in motion. Regarding dynamic stability evaluation, the authors focused on postural responses recorded via the center of pressure (COP) from underfoot force plates in different conditions, mostly providing disturbances while standing on a movable platform, then executing some balance tests, and performing overground gait trials. The results achieved were contradictory: generally showing either a decrease in postural equilibrium with pregnancy advancement, particularly in the third trimester, or an improvement of balance in late pregnancy. Let us briefly describe the main outcomes.
McCrory et al. [30] observed significantly smaller magnitude and velocity responses to perturbations during the third trimester compared to the second trimester or when compared with the control subjects (p < 0.05). While it may be argued that dynamic stability was improved in the third trimester, the authors suggest that the reduction of sway could also indicate a relative increase in torso rigidity leading to a greater risk of falling. In their second study [42], where they divided gravidas into those who experienced falls during pregnancy and those who did not, no differences were found between the nonpregnant control women and pregnant females who did not report a fall in any of the four dynamic stability variables. However, pregnant fallers demonstrated significantly less movement responses (i.e., sway velocity, and total sway) than pregnant non-fallers and the control group (p < 0.001). This could reflect an altered control system where ankle plantar flexors and dorsiflexors do not appropriately respond to control ankle joint torques.
Ersal et al. [43] used the scalar difference between the center of pressure and center of gravity (COP–COG) as a metric to characterize postural control and margin of stability assessment. Their experimental data indicated that pregnant fallers had significantly smaller peak COP–COG values compared with pregnant non-fallers and controls (p < 0.01), which may reflect the inability to generate adequate corrective torque in response to surface perturbations. This interpretation is in line with theoretical results indicating that pregnant non-fallers had higher ankle stiffness compared with pregnant fallers and controls, suggesting that ankle stiffness itself may be the dominant reason for the different dynamic response characteristics observed. The authors concluded that increasing ankle stiffness could be an important strategy to prevent falling by pregnant women.
Inanir [41] observed a decrease in postural equilibrium, particularly in the third trimester. All postural stability and fall risk test scores were significantly higher in the third trimester compared to the control group (p < 0.05). The third trimester indices for overall and medial-lateral stability index were significantly higher than the first and control groups (p = 0.001). Anterior-posterior stability index was greater in the third trimester compared to the control group. Additionally, the third-trimester fall risk test was significantly higher than in all the other groups (p < 0.001). The authors emphasized that dynamic postural stability indices may be used to predict measurements of postural equilibrium during pregnancy and thus evaluate the risk of falling.
El Shamy et al. [47] used the Biodex Balance System (BBS) at levels 7 and 8 and, similar to Inanir et al. [41], observed a decreased postural equilibrium in the third trimester compared with the second trimester of pregnancy. At both stability levels, the mean overall, anterior-posterior, and medial-lateral scores were significantly higher in the third trimester compared to the second trimester (p < 0.05). However, when comparing the scores between stability levels 8 and 7, there was no significant difference (p > 0.05) in the medial-lateral direction between the second and third trimesters of pregnancy.
Surprisingly, the results by Yoo et al. [36] revealed that the balance of pregnant women (both eyes open and eyes closed) increased toward the third trimester of pregnancy due to a significant decrease (p < 0.05) in the weight distribution index (WDI). However, WDI scores measured using the Tetrax system in pregnant women were still lower than in the control group. This means that the balance of pregnant women in the third trimester is better than that in the second trimester but similar to that of nonpregnant women. Additionally, this study revealed that the gait speed of the gravidas was significantly (p < 0.05) reduced (from 113 cm/s in the second trimester to 102 cm/s in the third trimester) compared to nonpregnant women (125 cm/s). Also, the cadence of the gravidas in the second and third trimesters (109 steps/min and 98 steps/min, respectively) showed a significant decrease (p < 0.05) when compared to nonpregnant women (114 steps/min). These findings are consistent with the results of Moccellin & Driuso [40], who registered ground reaction forces in pregnant and control groups and found that women in the first trimester and throughout pregnancy presented a trend of decreasing levels of the peaks, representing a decrease in gait velocity.
Shingala et al. [39] observed a significant decrease in postural balance in the third trimester compared to the nonpregnant group due to an increase in the time taken to complete the Four Square Step Test (FSST) (p < 0.05). For the pregnant females, they registered 13.15 ± 0.81 s (mean ± SD), while for the controls. 8.45 + 0.82 s. It showed that pregnant females took longer time to complete the test suggesting that their postural balance is compromised when compared to the control group.
4. Discussion
4.1. Principal findings
Postural stability is essential for functional independence in the pregnant population. As our study revealed, the postural control of gravid women in static conditions generally changed significantly throughout pregnancy only when visual cues were limited or the area of support was reduced. However, a few studies reported no changes in postural alignment. Importantly, research results indicate that women in advanced pregnancy may be at increased risk of falling when their vision is compromised. The results of research on dynamic stability are also inconclusive, with studies observing either a decrease in postural equilibrium with pregnancy advancement, particularly in the third trimester, or an improvement in balance in late pregnancy.
We should not be surprised by such unequivocal results, considering the complexity of integrating physiological mechanisms, processing sensory information in accordance with the postural body scheme during both standing and movement, the goals of the subjects, and their previous experience [3].
4.2. Larger oscillations of COP—what does it mean?
In terms of static and dynamic stability, researchers can be divided into two groups: those who recognize larger oscillations of COP as a tendency toward instability (due to factors like increased joint laxity and unevenly distributed mass around the body) [29, 32, 40, 52], and those who consider the postural control system adaptive to changes that occur during pregnancy and postpartum (mostly as changes in AP posture alignment and wider stance area) [48].
Supporters of the first interpretation suggest that, in terms of static stability, the increase of AP COP oscillations for pregnant women in the third trimester is due to increased ligament laxity [25, 54–56], which can cause greater ankle joint instability. Another hypothesis concerns the activity of the soleus muscle [37]. As revealed in the study by Moreira et al. [37], soleus EMG was not statistically different between control and pregnant groups (a similar proportion of motor neurons was recruited in both groups). Given that increased mass in pregnant women enhances the toppling torque, together with similar soleus activity, it would result in a more unstable system in the AP direction [57]. However, this hypothesis requires further exploration.
In dynamic conditions, Ersal et al. [43] identified an increase in ankle stiffness in pregnant non-fallers, which could be a response to pregnancy-related changes such as increased mass and ligament laxity [58, 59], or decreased nerve conduction velocity [60] and neuromuscular coordination [29, 35] during gestation. On the other hand, McCrory et al. [30] found alterations to dynamic stability in response to a translation postural perturbation in the third trimester: while reaction time was not affected by pregnancy, the amount of sway following the perturbation was reduced. They consider the relative stiffness of the torso to be the underlying cause. Wu et al. [61] speculated that increasing rigidity of the torso could be related to the higher incidence of falls in pregnant women. Another study by McCrory et al. [42] seems to confirm this view, showing that women who had not fallen demonstrated similar COP movement patterns during translational perturbations to nonpregnant women. However, pregnant women who reported falling presented decreased movement of the COP. This could reflect an altered control system, resulting in greater COM movement and potentially leading to a fall. The altered response to perturbation in pregnant fallers may also result from factors not assessed in this study, such as muscle strength.
As mentioned above, some authors consider the alterations in the postural control system as an adaptation to the changes that occur during pregnancy. To provide stability while standing, a pregnant woman adapts her posture by increasing lumbar lordosis and slightly tilting her body posteriorly [62]. Whitcome et al. [26] revealed that gravidas self-positioned in the natural stance maintain an almost constant center of mass position. Opala–Berdzik et al. [48] used this explanation for their results, which demonstrated unchanged AP static stability during the perinatal period, suggesting that in healthy women, the postural control system adapts to the changes that occur during pregnancy and postpartum.
At the same time, lateral stability is preserved throughout pregnancy because of the adaptive increase in stance width, which improves lateral balance [27, 35, 63]. The reason for the lack of lateral sway changes may be that a pregnant woman’s body shape changes evenly in the frontal plane, and the increasing mass is more equally distributed compared to the sagittal plane [48]. Other possible factors explaining higher stability in the ML direction are increased lumbar muscle activity [37] or the enlargement of the pelvis during pregnancy [27].
Besides bodily changes, behavioral factors may be relevant, as fear of falling can increase levels of caution in pregnant women, influencing their gait patterns. This effort to maintain equilibrium can provoke changes in the walking patterns of pregnant women [64]. One strategy used by pregnant women to maintain stability in both static and dynamic postures is repositioning their feet on the ground to increase their bases of support [62, 65–67]. Furthermore, Moccelin & Driuso [40] found that pregnant women tried to maximize their postural stability and control of sideways movements by adjusting their step width. This strategy requires adopting walking patterns that produce changes in the joint segments and lower limb muscles. Moccelin & Driuso [40] found a trend toward reduced postural control during the first trimester of pregnancy, with a further reduction by the third trimester, although no significant difference was found. Compared to the control group, pregnant women had larger COP displacement areas, longer time in the first phase of weight acceptance (the first task of the stance phase during the gait cycle, which comprises three functional demands: shock absorption, initial limb stability, and the preservation of progression [68]), lower values of the first and second peaks of the vertical component, and lower maximum and minimum values of the AP horizontal component of the GRF, possibly indicating a reduction in gait speed and thrust in the terminal stance phase of the gait cycle.
Data from Yu et al. [49] suggest that the effects of visual tasks on postural sway in the AP axis and stance width on postural sway in the ML axis can be seen during the first trimester of pregnancy. Like Jang et al. [27], they found effects on both the magnitude and dynamics of postural sway. Variations in visual tasks influenced both the magnitude of sway, as reflected in the positional variability of the COP, and the temporal dynamics of sway, as revealed by detrended fluctuation analysis of the COP data. Manipulation of stance width also influenced both the magnitude and dynamics of sway [49]. An increased amount of variability has been reported as a predictor of the risk of falling, with the assumption that it equates to increased instability. On the other hand, some evidence shows that increased variability may not be synonymous with dysfunction (i.e., postural stability decrease), implying that a moving system (e.g., a swaying body during posture or a moving body during gait) with large variability does not necessarily indicate either a highly stable system or poor stability [69].
4.3. Clinical applications
Health education programs are expected to be provided to future mothers so that women can improve their knowledge, attitudes, and skills for a healthy pregnancy and delivery. The above findings can be used as basic data for health promotion programs aimed at maintaining sound daily activities in pregnant women.
4.4. Recommendations for physical activity for pregnant women
Although exercise is safe for both the mother and fetus, most women reduce their activity level during the first weeks of gestation [70]. Therefore, professionals should encourage women to initiate or continue exercising during a healthy pregnancy. They should be more aware of the benefits of postural training aimed at improving their body stability during pregnancy and the postpartum period [27, 32, 46, 49, 52]. Lateral stability is maintained during pregnancy, likely accomplished by increasing stance width; thus, exercises employing various sizes of support bases and different visual stimuli could serve as effective interventions to minimize the effects of gestation on the control of posture while standing [32]. Takeda et al. [45] emphasized educating pregnant women that balance ability decreases from the second to the third trimester. Considering the effective interventions in the elderly population to improve postural stability, Jang et al. [27] suggest dynamic and static balance training exercises, including Tai Chi and strength training. Based on findings by Opala-Berdzik et al. [50], postpartum women, at least up to 6 months after delivery, should follow exercise recommendations similar to those for pregnant women, particularly avoiding excessive stretching that predisposes them to joint hypermobility. Given the different proprioception and kinesthetic feedback from looser connective tissue structures resulting in altered postural control, they recommend exercises that increase pelvis-spine complex stability in postpartum women. Sensory information from mechanoreceptors on the foot soles is an important part of postural control in quiet standing [71]. The study by Ramachandra et al. [72] suggests that ankle proprioception is significantly affected in the third trimester and does not return to baseline even at six weeks postpartum. This could be due to altered proprioceptive input from lax ligaments around the ankle joint and possibly mild edema, more common during the third trimester. Therefore, Ramachandra et al. [72] recommend that lower limb joint proprioceptive training, especially for the ankle joint, should be part of antenatal and postnatal exercises. Shingala et al. [39] propose a balance exercise program that can be added to the Ante-Natal program for pregnant women, with proper assessment and supervision. Takeda et al. [46] suggest employing Whipple’s [73] recommendations, emphasizing effective posture control training features: 1) body-weight exercises; 2) interactions between the body and head (eyeball movement), including quick horizontal movements; and 3) activation of muscle groups, including amplitude motion in the vertical direction and the thigh and hip joints. Additionally, Takeda et al. [46] recommend incorporating anti-gravity movements to promote muscle activity supporting body weight, being conscious of “wobble” in all directions due to weight shifts. Recognizing displacement due to changes in weight and the body’s center of gravity as pregnancy progresses is important. McCrory et al. [42], who observed that all sedentary pregnant women in their study fell during their pregnancies, suggest that exercise may play a role in fall prevention in pregnant women. Moccellin & Driuso [40] and Sancar et al. [44] encourage therapists to directly address the postural control of pregnant women and provide physical activity programs aimed at improving balance, maintaining muscle tone and strength, preventing falls, and promoting physical well-being during pregnancy. They should also educate pregnant women about pain relief and correct movement methods to help them cope with physical and functional changes due to pregnancy [36].
4.5. Methodological appraisal of the included studies: Strengths and limitations
We have identified some areas of weakness that should be addressed in future studies:
4.5.1. Sample details.
Information about the sample groups should be more detailed, including:
The recruitment process of participants, which could indicate to what extent the sample is representative and whether the findings can be generalized.
The process of participant exclusion (how many and when they were excluded), which could potentially explain the small sample sizes in some studies.
4.5.2. Lack of homogeneity in study groups.
The number of subjects varied across studies, with some studies having small sample sizes (<20).
The division into trimesters varied between studies (assignment to a specific trimester based on the week of pregnancy). Additionally, some studies lacked information about the week of pregnancy when the data were collected.
The lack of homogeneity in study groups may partially explain the inconclusive research results. Inconsistencies in trimester division create difficulties and reduce the credibility of comparative analysis, as women described to be at the same stage of pregnancy are assigned to trimesters according to different criteria.
4.5.3. Limited longitudinal analyses.
Among the analyzed studies, only six included longitudinal analysis of postural stability throughout the three trimesters of pregnancy. Longitudinal studies are demanding, requiring participants to attend multiple laboratory sessions. Consequently, some authors opted out of this approach and focused only on the most advanced period of pregnancy, using nulliparous women as a control group or comparing women at various pregnancy stages at a single time point.
4.5.4. Methodology and postural indices.
The time duration of trials varied greatly across studies, and only one study [38] met the Ruhe et al. [74] recommendation of ≥90 seconds. Longer recording times are necessary to observe the development of certain postural behaviors in bipedal standing [75]. Short trial times may not allow enough time to observe the actual postural strategies of individuals.
Including multiple indices of postural sway is beneficial for identifying clearer patterns of postural behavior. Although many parameters were included in the analyzed studies, few were repeatable, making comparisons between studies difficult. The equipment used also varied greatly.
The importance of stance width for stability measures has been shown; however, some studies did not include information about it (preferred/fixed stance width) in the body position description [29, 30, 40, 41].
4.5.5. Precautions related to pregnancy.
Due to precautions related to pregnancy, the perturbations and tasks used while testing dynamic stability may be too small to induce sufficient instability to cause stepping or loss of balance [30, 42].
Strengths of the Analyzed Studies:
- Examination of direction-specific balance measures [27].
- The majority of the studies (14) included information about parity. One study also included information about gravidity and showed that the groups were matched for parity/gravidity [41].
- A few studies used a longitudinal design, allowing observation of the dynamics of changes during pregnancy.
- Most of the papers provided not only a diagnosis of the state but also an explanation of the results achieved.
4.6. Strengths and limitations
The primary strength of our review lies in the identification of specific methodological weaknesses within the analyzed studies. Addressing these weaknesses in future research may lead to improved research designs and greater consistency in findings. Additionally, our review encompasses studies evaluating postural stability under both static and dynamic conditions, thereby extending the existing body of literature [53]. We have elucidated the strategies employed by pregnant females to enhance body stability, which adds a significant contribution to understanding postural adaptations during pregnancy. Importantly, we have provided recommendations and clinical implications derived from these studies aimed at reducing the risk of falls in the pregnant population. Moreover, we emphasize the limitations of the quiet stance paradigm in evaluating postural sway. This paradigm is limited in its ability to detect and control postural sway with reference to other behaviors. Traditional tests of quiet stance are not representative of real-world conditions, where individuals are often engaged in other activities while standing, such as reading, manual manipulation, or visual tracking. Therefore, future studies on postural stability throughout pregnancy should consider that postural control is integrated with the execution of suprapostural activities.
Supporting information
S1 Table. Search strategy used in SportDiscuss with full text and MEDLINE database.
https://doi.org/10.1371/journal.pone.0312868.s002
(DOCX)
S2 Table. Search strategy used in Health Source—Consumer Edition and Health Source: Nursing/Academic Edition and Rehabilitation & Sports Medicine Source database.
https://doi.org/10.1371/journal.pone.0312868.s003
(DOCX)
S3 Table. Search strategy used in PubMed database.
https://doi.org/10.1371/journal.pone.0312868.s004
(DOCX)
S4 Table. Modified downs and black checklist for quality assessment—Part 1.
https://doi.org/10.1371/journal.pone.0312868.s005
(DOCX)
S5 Table. Modified downs and black checklist for quality assessment—Part 2.
https://doi.org/10.1371/journal.pone.0312868.s006
(DOCX)
S6 Table. All studies identified in the literature search, including those that were excluded from the analyses with the reason.
https://doi.org/10.1371/journal.pone.0312868.s007
(DOCX)
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