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Abstract
Objective Gestational weight gain (GWG) has been identified as a critical modifier of maternal and fetal health. This systematic review and meta-analysis aimed to examine the relationship between prenatal exercise, GWG and postpartum weight retention (PPWR).
Design Systematic review with random effects meta-analysis and meta-regression. Online databases were searched up to 6 January 2017.
Study eligibility criteria Studies of all designs in English, Spanish or French were eligible (except case studies and reviews) if they contained information on the population (pregnant women without contraindication to exercise), intervention (frequency, intensity, duration, volume or type of exercise, alone [“exercise-only”] or in combination with other intervention components [eg, dietary; “exercise + co-intervention”]), comparator (no exercise or different frequency, intensity, duration, volume or type of exercise) and outcomes (GWG, excessive GWG (EGWG), inadequate GWG (IGWG) or PPWR).
Results Eighty-four unique studies (n=21 530) were included. ‘Low’ to ‘moderate’ quality evidence from randomised controlled trials (RCTs) showed that exercise-only interventions decreased total GWG (n=5819; −0.9 kg, 95% CI −1.23 to –0.57 kg, I2=52%) and PPWR (n=420; −0.92 kg, 95% CI −1.84 to 0.00 kg, I2=0%) and reduced the odds of EGWG (n=3519; OR 0.68, 95% CI 0.57 to 0.80, I2=12%) compared with no exercise. ‘High’ quality evidence indicated higher odds of IGWG with prenatal exercise-only (n=1628; OR 1.32, 95% CI 1.04 to 1.67, I2=0%) compared with no exercise.
Conclusions Prenatal exercise reduced the odds of EGWG and PPWR but increased the risk of IGWG. However, the latter result should be interpreted with caution because it was based on a limited number of studies (five RCTs).
- pregnancy
- body weight regulation
- exercise
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Introduction
Over the past three decades, gestational weight gain (GWG) has emerged as a critical modifier of maternal and fetal health during pregnancy that persists for years after delivery.1 Compared with adequate GWG (AGWG), excessive GWG (EGWG) and inadequate GWG (IGWG) have been associated with higher adverse maternal and infant outcomes. EGWG was associated with higher odds of large for gestational age (LGA) babies, macrosomia and caesarean delivery.2 IGWG was associated with increased odds of small for gestational age (SGA) babies and preterm birth; these associations were greatest with lower pre-pregnancy body mass index (BMI).2 Several studies have also highlighted that EGWG may contribute exponentially to the obesity epidemic in women.3 4 Taken together, these findings highlight the importance of identifying and implementing prenatal strategies that promote AGWG. In fact, one of the initiatives for ‘’Healthy People 2020’' is to increase the proportion of pregnant women who achieve AGWG.5
A recent meta-analysis of 33 randomised controlled trials (RCTs; n=9320 women) conducted by the International Weight Management in Pregnancy Collaborative Network showed that diet- and physical activity-based interventions reduced GWG by an average of 0.70 kg (95% CI 0.92 to 0.48) compared with the control group.6 Likewise, the most recent Cochrane review reported a risk reduction of EGWG with diet or exercise, or both interventions (overall result: 24 RCTs, n=7096; average RR 0.80, 95% CI 0.73 to 0.87).7 However, none of these meta-analyses examined whether a dose–response relationship existed between physical activity and GWG or whether prenatal physical activity impacted postpartum weight retention (PPWR).
International and national guidelines for exercise during pregnancy recommend that women without contraindications should be physically active throughout pregnancy.8 9 The present systematic review and meta-analysis was conducted as part of a series of reviews which form the evidence base for the development of the 2019 Canadian Guideline for Physical Activity throughout Pregnancy (herein referred to as the Guideline).10 The purpose of this review was to evaluate the effect of prenatal exercise on GWG and PPWR.
Methods
In October 2015 the Guidelines Consensus Panel assembled to identify the priority outcomes for the Guideline update. The Panel included researchers, methodological experts, a fitness professional and representatives from the Canadian Society for Exercise Physiology (CSEP), the Society of Obstetricians and Gynaecologists of Canada (SOGC), the College of Family Physicians of Canada, the Canadian Association of Midwives, the Canadian Academy of Sport and Exercise Medicine, Exercise is Medicine Canada and a representative health unit (the Middlesex-London Health Unit). Twenty ‘critical’ and 17 ‘important’ outcomes related to prenatal exercise and maternal or fetal health were selected by the Panel. Two of the ‘critical’ outcomes (ie, EGWG and PPWR) and two of the ‘important’ outcomes (ie, GWG and IGWG) are examined in this review. The Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement and checklist were used to guide this systematic review and meta-analysis.11
Protocol and registration
Two systematic reviews were undertaken to examine the impact of prenatal exercise on fetal and maternal health outcomes, respectively, and records identified through both processes were considered for inclusion in the current review. Each review was registered a priori with the International Prospective Register of Systematic Reviews (PROSPERO; fetal health: CRD42016029869; maternal health: CRD42016032376). Since the relationships between prenatal exercise and maternal/fetal health outcomes were examined in studies related to both maternal and fetal health, records retrieved from both of these searches were evaluated for inclusion in the current review.
Eligibility criteria
This study was guided by the participants, interventions, comparisons, outcomes and study design (PICOS) framework.12
Population
The population of interest was pregnant women without contraindication to exercise (according to the CSEP and American College of Obstetricians and Gynaecologists guidelines).8 13 Absolute contraindications to exercise were defined as: ruptured membranes, premature labour, persistent second or third trimester bleeding, placenta previa, pre-eclampsia, gestational hypertension, incompetent cervix, intrauterine growth restriction, high order pregnancy, uncontrolled type 1 diabetes, hypertension or thyroid disease or other serious cardiovascular, respiratory or systemic disorders. Relative contraindications to exercise were defined as: a history of spontaneous abortion, premature labour, mild/moderate cardiovascular or respiratory disease, anaemia or iron deficiency, malnutrition or eating disorder, twin pregnancy after 28 weeks or other significant medical conditions.8 13
Intervention (exposure)
The intervention/exposure of interest was objectively or subjectively measured prenatal exercise of any frequency, intensity, duration, volume or type (studies on exercise during labour were not eligible for inclusion). Although exercise is a subtype of physical activity, the terms are used interchangeably in this review. Exercise was defined as any bodily movement generated by skeletal muscles that resulted in energy expenditure above resting levels.14 Interventions that consisted of exercise alone (termed ‘exercise-only’ interventions) or exercise combined with other interventions (eg, diet; termed ‘exercise + co-interventions’) were considered. Prenatal exercise could be acute (ie, a single exercise session) or habitual (ie, usual activity).
Comparison
Eligible comparators were: no exercise; different frequency, intensity, duration, volume or type of exercise; different intervention duration; or exercise in a different trimester.
Outcomes
Relevant outcomes were total GWG, GWG and weekly GWG during the intervention time frame, EGWG, AGWG, IGWG and PPWR. Weekly GWG during the intervention was defined as the amount of weight the women gained during the intervention time frame divided by the duration of the intervention (in weeks); EGWG, AGWG and IGWG were defined according to pre-pregnancy BMI of the women using either the 1990 or 2009 Institute of Medicine (IOM) GWG recommendations (see table 1 for more details about the recommendations)15 16; and PPWR was defined as postpartum body weight minus pre-pregnancy body weight.
Institute of Medicine (IOM) gestational weight gain recommendations15 16
Study design
Primary studies of any design were eligible, with the exception of case studies (n=1), narrative syntheses and systematic reviews.
Information sources
A comprehensive search was created and run by a research librarian (LGS) in the following databases: MEDLINE, EMBASE, PsycINFO, Cochrane Database of Systematic Reviews, Cochrane Central Register of Controlled Trials, Scopus and Web of Science Core Collection, CINAHL Plus with Full-text, Child Development & Adolescent Studies, ERIC, Sport Discus, ClinicalTrials.gov and the Trip Database up to 6 January 2017 (see online supplement for complete search strategies).
Supplementary file 1
Study selection and data extraction
Two independent reviewers screened the titles and abstracts of all articles identified in the search against the inclusion criteria. Abstracts that were selected as eligible at level 1 by at least one reviewer were retrieved for level 2 screening as a full-text article. Two independent reviewers screened full-text articles against the study inclusion criteria. When a study was recommended by one or more reviewers for exclusion, further review was conducted by MHD and/or SMR for a final decision. If a decision could not be made, the study characteristics were presented to the Guidelines Steering Committee who oversaw the systematic reviews (MHD, MFM, SMR, CG, VP, AJG and NB) and a final decision regarding inclusion/exclusion was made by consensus. Studies identified by the maternal and fetal search strategies were imported into DistillerSR for de-duplication and data extraction, and were subsequently considered as one review.
Following consultation with methodological experts and the Guidelines Steering Committee, data extraction was completed in DistillerSR and data extraction tables were created. Data from records that met the inclusion criteria were extracted by one person and independently verified by a content expert (MHD, MFM or SMR). For studies where multiple publications existed, the most recent or complete publication was selected as the ‘parent’ paper; however, relevant data from all publications were extracted. Extracted data included study characteristics (ie, year, study design, country), characteristics of the population (eg, number of participants, age, pre-pregnancy BMI, parity and pregnancy complications including pre-eclampsia, gestational hypertension and gestational diabetes), intervention/exposure (prescribed and/or measured exercise frequency, intensity, duration, type and volume, intervention duration, measurement tool), and outcomes (GWG, EGWG, AGWG, IGWG and PPWR). If data were unavailable for extraction, authors were contacted to request additional information. See online supplementary table 1 for included study characteristics.
Quality of evidence assessment
The Grading of Recommendations Assessment, Development and Evaluation (GRADE) framework was used to assess the quality of evidence across studies for each study design and health outcome.17
Accordingly, evidence from RCTs was considered ‘high’ quality and was graded down if there was a concern with risk of bias,18 indirectness,19 inconsistency,20 imprecision21 or risk of publication bias22 because these factors reduce the level of confidence in the observed effects. Evidence from all non-randomised interventions and observational studies began with a ‘low’ quality rating and, if there was no cause to downgrade, was upgraded if applicable according to the GRADE criteria (eg, large magnitude of effect, evidence of dose-response).23 Specifically, the risk of bias in RCTs and intervention studies was assessed following the Cochrane Handbook24 and the risk of bias in observational studies was assessed using the characteristics recommended by Guyatt et al,18 consistent with systematic reviews conducted to support previous health behaviour guidelines.25 26 All studies (RCTs, non-randomised intervention studies and observational studies) were assessed for potential sources of bias including selection bias (RCT/intervention: inadequate randomisation procedure; observational: inappropriate sampling), reporting bias (selective/incomplete outcome reporting), performance bias (RCT/intervention: compliance with the intervention; observational: flawed measurement of exposure), detection bias (flawed measurement of outcome), attrition bias (incomplete follow-up, high loss to follow-up) and ‘other’ sources of bias. Risk of bias across studies was considered ’serious’ when studies with the greatest influence on the pooled result (assessed using weight (%) given in forest plots or sample size in studies that were narratively synthesised) presented ‘high’ risk of bias. The greatest influence on the pooled result was determined as follows: the studies that had the greatest individual percent contribution in the meta-analyses, when taken together, contribute to >50% of the weight of the pooled estimate. Additionally, studies were considered to reflect a serious risk of bias when the sample size of narratively synthesised studies was similar to the total sample size of studies contributing to >50% of the weight of the pooled estimate in the meta-analyses. Given the nature of exercise interventions, it is not possible to blind participants to group allocation, and selection risk of bias was rated as ‘low’ if this was the only source of bias identified. Performance bias was rated as ‘high’ when <60% of participants performed 100% of prescribed exercise sessions or attended 100% of counselling sessions (defined as low compliance) or when compliance to the intervention was not reported. Attrition bias was rated as ‘high’ when >10% of data were missing at the end of the study and intention-to-treat analysis was not used.
Inconsistency across studies was considered serious when heterogeneity was high (I2 ≥50%) or when only one study was assessed (I2 unavailable). Indirectness was considered serious when interventions included both exercise and additional components (ie, exercise + co-interventions, or exercise-only and exercise + co-interventions combined in analyses). Imprecision was considered serious when the 95% CI crossed the line of no effect and was wide, such that interpretation of the data would be different if the true effect was at one end of the CI or the other. When only one study was assessed, imprecision was not considered serious because inconsistency was already considered serious for this reason. Finally, when possible (ie, at least 10 studies were included in the forest plot), publication bias was assessed via funnel plots (see online supplement). If there were fewer than 10 studies, publication bias was deemed non-estimable and not rated down. Original plans for two people to independently assess the quality of the evidence across each health outcome were amended for feasibility reasons. As such, one reviewer evaluated the quality of the evidence and a second person checked the GRADE tables as a quality control measure. GRADE tables are presented in online supplementary tables 2–8.
Statistical analysis
Statistical analyses were conducted using Review Manager v5.3. (Cochrane Collaboration, Copenhagen, Denmark). Odds ratios were calculated for all dichotomous outcomes. Inverse-variance weighting was applied to obtain OR using a random effects model. For continuous outcomes, mean differences (MD) between exercise and control groups were calculated. When applicable, change scores were calculated using the generic inverse variance method (Cochrane Collaboration, Copenhagen).27 Significance was set at P<0.05.
A staged approach was used to determine if there was sufficient evidence from high-quality study designs (ie, RCTs) to inform the Guideline, or if evidence from lower quality study designs needed to be examined. If meta-analyses of RCTs contained data from fewer than 2000 women, the impact of prenatal exercise on the specific outcome was examined further using observational evidence (first non-randomised interventions; if 2000 women were not reached after adding non-randomised interventions, we used cohort, cross-sectional and case–control studies).
Meta-analyses were performed separately by study design. For RCTs and non-randomised interventions, sensitivity analyses were performed to evaluate whether the effects were different when examining the relationships between exercise-only interventions (including standard care) and exercise + co-interventions and the outcomes of interest. When possible, the following a priori determined subgroup analyses were conducted for exercise-only interventions and observational studies: (1) women diagnosed with diabetes (gestational, type 1 or type 2) compared with women without diabetes (named ‘general population’); (2) women with pre-pregnancy overweight or obesity status (mean BMI >25.0 kg/m2) compared with women who were of various BMI (mean BMI <25 kg/m2, which may include some individuals with BMI >25.0 kg/m2; named ‘general population’); (3) women >35 years of age compared with women <35 years of age; (4) women who were previously inactive compared with those who were previously active (as defined by individual study authors). If a study did not provide sufficient detail to allow for inclusion into the a priori subgroups, then a third group called ‘unspecified’ was created. A priori subgroup analyses were also conducted for exercise-only RCTs to identify whether relationships between exercise and outcomes differed depending on the type of exercise (eg, aerobic exercise, resistance training or yoga). Due to feasibility, these subgroup analyses were only conducted for outcomes rated as ‘critical’. Finally, a posteriori analyses were conducted for exercise-only RCTs to examine whether the use of the 1990 or 2009 IOM GWG recommendations affected the results for EGWG, AGWG and IGWG. Tests for subgroup differences were conducted, with statistical significance set at P<0.05. Effects within subgroups were only interpreted when statistically significant differences between subgroups were found. The I2 was calculated for the overall meta-analyses and individual subgroup analyses to indicate the percent of total variability that was attributable to between-study heterogeneity. In studies where there were no observed events in the intervention or control group, data were entered into forest plots, but were considered ‘not estimable’ and excluded from the pooled analysis as per the recommendation in the Cochrane Handbook.28
In order to identify a clinically meaningful decrease in EGWG and IGWG, dose–response meta-regression29–31 was carried out by weighted no-intercept regression of log OR with a random effects for study, using the metafor32 package in R (version 3.4.1).33 It was determined that an accepted cut-off point for a clinically meaningful decrease does not exist in the literature. As such, a reduction of 25% was chosen based on expert opinion. Models did not include an intercept term since the log OR is assumed to be zero when the exercise dose is zero. Restricted cubic splines with knots at the 10th, 50th and 90th percentiles of the explanatory variable34 were used to investigate whether there was evidence for a non-linear relationship. Fitting was performed by maximum likelihood, and non-linearity was assessed using a likelihood ratio test. When the model was statistically significant at P<0.05, the minimum exercise dose to obtain a clinically significant benefit was estimated by the minimum value of the explanatory variable at which the estimated OR was less than 0.75.
For outcomes or for subsets of studies where a meta-analysis was not possible, a narrative synthesis of the results was presented by study design, organised around each outcome. Unless otherwise specified, studies were not included in meta-analyses if data were reported incompletely (eg, SD, SE or number of cases/controls not provided), if data were adjusted for confounding factors, or if the study did not include a non-exercise control group. In studies where data were included in the meta-analysis but additional information was available that could not be meta-analysed, the studies were included in both the meta-analysis and narrative synthesis.
Results
Study selection
Althought the initial search was not limited by language, the Guidelines Steering Committee decided to exclude studies published in languages other than English, Spanish or French for feasibility reasons. A PRISMA diagram of the search results, including reasons for exclusion, is shown in figure 1. A comprehensive list of excluded studies is presented in the online supplement.
Flow diagram of studies selected for the present study. Among the 84 studies included, 74 were included in quantitative synthesis, but three were from the same study and counted as one unique study; 19 studies were included in qualitative synthesis, but six were from the same study and counted as one unique study; 16 studies were included in both the qualitative and quantitative synthesis.
Study characteristics
Overall, 84 unique studies (n=21 530) from 26 countries were included. There were 79 RCTs (46 exercise-only interventions and 33 exercise + co-interventions), four non-randomised interventions and one cohort study. The co-interventions included diet and behavioural interventions to improve eating and physical activity habit and/or limit GWG. Among the exercise-only interventions, the frequency of the prescribed exercise ranged from 1 to 7 times per week. The majority of the studies targeted moderate-intensity exercise (Borg scale 12–14/20). The duration of each exercise session ranged from 10 to 90 min. The types of exercise were walking, swimming, stationary cycling, water gymnastics, resistance training, stretching, yoga or pelvic floor muscle training. The majority of the interventions started before 20 weeks of gestation with most of the studies ending in the mid-to-late third trimester. Additional details about the studies can be found in the online supplement.
Quality of evidence
Overall, the quality of evidence ranged from ‘very low’ to ‘high’ (see online supplementary tables 2-8). The most common reasons for downgrading the quality of evidence were (1) serious risk of bias; (2) inconsistency; and (3) indirectness of the interventions being assessed. Common sources of bias included poor or unreported compliance with the intervention and inappropriate treatment of missing data when the attrition rate was high. No evidence of publication bias was observed.
Synthesis of data
Excessive gestational weight gain
There was ‘low’ quality evidence from 33 RCTs (n=9138) regarding the association between prenatal exercise and EGWG.35–67 The quality of evidence was downgraded from ‘high’ to ‘low’ because of serious risk of bias and serious indirectness of the interventions. The pooled estimate based on 32 RCTs (n=8817) indicated 32% lower odds of EGWG with exercise compared with no exercise (OR 0.68, 95% CI 0.59 to 0.78, I2=46%; figure 2).35–66 One exercise + co-intervention could not be included in the meta-analysis and showed that the proportion of women gaining above GWG recommendations did not differ between the groups (co-intervention group, n=158; control group, n=163) (RR 1.06, 95% CI 0.83 to 1.36; see online supplementary table 1).67
Effects of prenatal exercise-only or exercise plus co-interventions (RCTs) compared with control on odds of excessive gestational weight gain. Analyses conducted with a random effects model. CI, confidence interval; df, df; M-H, Mantel–Haenszel method. NW: subgroup of women with normal weight; OW/OB: subgroup of women with overweight/obesity; OB: subgroup of women with obesity; Exercise: exercise arm of the intervention; Diet + exercise: diet + exercise arm of the intervention.
The results of the meta-regression analysis are presented in online supplementary figures 31–34). To achieve at least a 25% reduction in the odds of EGWG, pregnant women needed to exercise at least two times per week, 35 min/session or accumulate at least 456 MET-min/week of moderate intensity exercise (eg, 105 min of brisk walking, water aerobics, stationary cycling or resistance training per week).
Five RCTs included in the pooled results reported additional data that could not be included in the meta-analysis (see online supplementary table 1).41 47 60 62 68 Barakat et al 47 and Ruiz et al 41 reported 37–47% lower odds of EGWG with exercise-only interventions (after adjustment for several potential confounding factors) whereas Renault et al 68 found no effect of an exercise-only intervention (OR 0.86, 95% CI 0.68 to 1.08) compared with no exercise. In contrast, they found that an exercise + diet intervention reduced the odds of EGWG (OR 0.73, 95% CI 0.57 to 0.94).68 Phelan et al 62 also reported lower odds of EGWG with an exercise + diet intervention but only in women of normal pre-pregnancy weight (OR 0.38, 95% CI 0.20 to 0.87; women with overweight/obesity: OR 1.4, 95% CI 0.70 to 2.7). Finally, Kinnunen et al 60 reported no effect of an exercise + diet intervention on the odds of EGWG (OR 0.82, 95% CI 0.53 to 1.26) after adjustment for several potential confounding factors.
Sensitivity analysis
The pooled estimate for the exercise-only interventions was not significantly different from the pooled estimate for the exercise + co-interventions (P=0.89). Both exercise-only interventions and exercise + co-interventions reduced the odds of EGWG (figure 2). Specifically, there was ‘moderate’ quality evidence (downgraded due to serious risk of bias) indicating a 32% reduction in the odds of EGWG with exercise-only interventions (15 RCTs, n=3519; OR 0.68, 95% CI 0.57 to 0.80, I2=12%).35–49
Subgroup analysis
The tests for subgroup differences performed for exercise-only interventions were not significant (see online supplementary figures 2–5).
Inadequate gestational weight gain
There was ‘low’ quality evidence from 15 RCTs (n=5858) regarding the association between prenatal exercise and IGWG.39 41–43 45 53 55 60–62 64–67 69 The quality of evidence was downgraded from ‘high’ to ‘low’ because of serious risk of bias and serious indirectness of the interventions. The pooled estimate based on 12 RCTs (n=4848) showed 22% higher odds of IGWG with exercise compared with no exercise (OR 1.22, 95% CI 1.03 to 1.45, I2=11%; figure 3).39 41–43 45 55 60–62 64–66 Three RCTs could not be included in the meta-analysis (intervention group, n=662; control group, n=348) and showed no increased odds of IGWG with prenatal exercise compared with no exercise (see online supplementary table 1). Jackson et al 67 reported that approximately 14% of the women in both the exercise + co-intervention (diet and behavioural intervention) (n=158) and control groups (n=163) gained less than the GWG recommendations. Simmons et al 69 and Vinter et al 53 showed no increased odds of IGWG (ie, GWG <5 kg in women with obesity) with prenatal exercise, either alone or in combination with a diet intervention compared with no exercise. Moreover, one trial included in the pooled results reported additional data that found no significant relationship between exercise and IGWG.68
Effects of prenatal exercise-only or exercise plus co-interventions compared with control on inadequate gestational weight gain (RCTs). Analyses conducted with a random effects model. df, degrees of freedom; M-H, Mantel–Haenszel method. NW: subgroup of women with normal weight; OW/OB: subgroup of women with overweight/obesity; OW: subgroup of women with overweight; OB: subgroup of women with obesity; Exercise: exercise arm of the intervention; Diet + exercise: diet + exercise arm of the intervention.
The results of the meta-regression analysis are presented in online supplementary figures 35–38. Overall, the results suggested that a 25% increased risk of IGWG was attained when exercise frequency exceeded four times/week, exercise duration exceeded 40 min or exercise volume exceeded 550 MET-min/week.
Sensitivity analysis
The pooled estimate for the exercise-only interventions was not significantly different from the pooled estimate for the exercise + co-interventions (P=0.64). Specifically, there was ‘high’ quality evidence indicating that exercise-only interventions increased the odds of IGWG by 32% (pooled estimate based on five RCTs, n=1628; OR 1.32, 95% CI 1.04 to 1.67, I2=0%; figure 3).39 41–43 45 There was no statistically significant difference for those participating in exercise + co-interventions (figure 3).
Subgroup analysis
The tests for subgroup differences performed for exercise-only interventions were not significant (see online supplementary figures 7–10).
Postpartum weight retention
There was ‘very low’ quality evidence from 15 RCTs (n=4725) regarding the association between prenatal exercise and PPWR.44 45 55 56 59 61–63 70–76 The quality of evidence was downgraded from ‘high’ to ‘very low’ because of serious risk of bias, serious inconsistency and serious indirectness of the interventions. The pooled estimate based on 11 RCTs (n=4196) indicated lower PPWR with prenatal exercise compared with no exercise (−0.85 kg, 95% CI −1.46 to –0.25 kg, I2=60%; figure 4).44 45 55 56 61–63 70–74 Three RCTs (intervention group, n=268; control group, n=261) could not be included in the meta-analysis (see online supplementary table 1). In the first, Vinter et al 76 reported similar maternal weight at 6 weeks postpartum in women who were randomised to a prenatal exercise + diet intervention (95 kg, 87.3–106.6 kg) and in those who were randomised to a control group (94.4 kg, 87–107.3 kg, P=0.878). The proportion of women meeting a postpartum weight loss goal of >0 kg and >5 kg was also similar in both groups.76 Second, Ferrara et al 59 reported no effect of a prenatal exercise + co-intervention (diet and behavioural intervention) on the odds of meeting postpartum weight loss goals (goal based on pre-pregnancy BMI) at 6 weeks and 7 months postpartum. The results were the same when examining women with normal weight and those with overweight/obesity, or women who gained ≤ or > IOM GWG recommendations.59 However, data at 12 months postpartum showed that women who gained ≤ IOM GWG recommendations and who participated in a prenatal exercise + co-intervention were more likely to achieve weight loss goals than those who did not participate.59 Finally, a superiority trial showed similar maternal weight retention at 2 months postpartum in both exercising groups (low-intensity exercise: 5.4±3.9 kg vs moderate-intensity exercise: 4.6±3.3 kg), as well as similar numbers of women with PPWR <2 kg (low-intensity exercise: 18% vs moderate-intensity exercise: 28%).75 When the data were compared with a historical control cohort, women in the moderate-intensity exercise group had lower weight and were more likely to achieve PPWR of <2 kg.75
Effects of prenatal exercise-only or exercise plus co-interventions compared with control on postpartum weight retention (RCTs). Analyses conducted with a random effects model. df, degrees of freedom; IV, inverse variance; SD. Mean differences are in kg.
Five RCTs included in the pooled results reported additional data that could not be included in the meta-analysis55 56 63 70 72 77 (see online supplementary table 1). Specifically, the study by Althuizen et al 56 showed that prenatal exercise + co-intervention (diet and behavioural intervention) did not reduce the odds of retaining >5 kg at 12 months postpartum (OR 1.2, 95% CI 0.41 to 3.51). Similarly, Phelan et al reported no effect of prenatal exercise + co-intervention (diet and behavioural intervention) on the odds of returning to pre-pregnancy weight at 12 months postpartum (OR 1.6, 95% CI 0.92 to 2.7).72 In contrast, Sagedal et al 63 reported higher odds of returning to pre-pregnancy weight at 12 months postpartum (OR 1.50, 95% CI 1.01 to 2.24) in women who participated in an exercise + diet intervention. Similarly, Rauh et al reported lower odds of PPWR >5 kg at 4 months postpartum (OR 0.5, 95% CI 0.2 to 0.9, adjusted for several potential confounding factors)55 and at 12 months postpartum (OR 0.40, 95% CI 0.16 to 0.97, adjusted for age, pre-pregnancy BMI, time of 12-month follow-up and practice)77 with a prenatal exercise + diet intervention compared with no intervention. Finally, Seneviratne et al 70 found that compliance with an exercise-only intervention (ie, the percentage of prescribed exercised sessions completed) was associated with maternal postnatal BMI (beta −0.031, 95% CI −0.059 to −0.002, adjusted for ethnicity, parity and baseline BMI).
The results of the meta-regression analysis showed no dose–response relationship between prenatal exercise and PPWR.
Sensitivity analysis
The pooled estimate for the exercise-only interventions was not significantly different from the pooled estimate for the exercise + co-interventions (P=0.87). Specifically, there was ‘moderate’ quality evidence (downgraded due to serious risk of bias) showing that prenatal exercise-only interventions were associated with lower PPWR (three RCTs, n=420; −0.92 kg, 95% CI −1.84 to 0.00 kg, I2=0%; figure 4)44 45 70 compared with no exercise.
Subgroup analysis
The tests for subgroup differences performed for exercise-only interventions were not significant (see online supplementary figures 12–14).
The results for total GWG, GWG and weekly GWG during the intervention and AGWG are presented in the online supplement. In summary, prenatal exercise was associated with lower total GWG compared with no exercise (59 RCTs, n=13 180; see online supplementary figure 15) and a dose–response relationship was found between prenatal exercise duration, frequency and volume and total GWG (see online supplementary figures 39–42). Similarly, prenatal exercise was associated with lower GWG during the intervention (17 RCTs, n=2316; see online supplementary figure 20) and lower weekly GWG (five RCTs, n=1095; see online supplementary figure 24) compared with no exercise. Finally, there was a 39% greater odds of AGWG with exercise compared with no exercise (16 RCTs, n=5497; see online supplementary figure 26).
Discussion
In this comprehensive systematic review and meta-analysis of 84 studies there was ‘moderate’ quality evidence indicating 32% decreased odds of EGWG with exercise-only interventions (15 RCTs, n=3519).35–49 In order to achieve at least a 25% reduction in the odds of EGWG, pregnant women needed to accumulate at least 456 MET-min/week of moderate intensity exercise (eg, 105 min of brisk walking, water aerobics, stationary cycling or resistance training per week). Results from meta-regression analyses suggested that prevention of EGWG was attained when exercise was performed at a frequency of at least 2 days per week and/or at least 35 min per session. Accumulating higher exercise volume was associated with a greater reduction in the odds of EGWG. EGWG is associated with long-term maternal and fetal overweight/obesity, cardiovascular morbidity and mortality.3 78–80 Therefore, the observed 32% reduction in odds of EGWG indicates that prenatal exercise may be an effective low-cost strategy to promote AGWG, and may have important implications for the long-term health of two generations. However, a better understanding of factors influencing the efficacy of prenatal exercise interventions targeting GWG in overweight and obese women is needed.
We also found ‘low’ to ‘moderate’ quality evidence indicating that exercise-only interventions were associated with a 1 kg reduction in both total GWG (28 RCTs, n=5819)35–37 41 43–45 47 49 70 81–98 and PPWR (three RCTs, n=420).44 45 70 From a clinical standpoint, a 1 kg difference in total GWG is small, and would only be meaningful if it was associated with higher adverse maternal and infant outcomes. Our data indicated higher odds of IGWG with prenatal exercise. A clinically meaningful increase in the odds of IGWG was attained when exercise volume exceeded 550 MET-min/week. Our findings therefore suggest that women who accumulated at least 550 MET-min/week of moderate-intensity exercise had reduced odds of gaining excessively but also had increased odds of not gaining enough weight. It is important to note that the results for IGWG were only based on five studies. We must acknowledge the uncertainty around the meaningfulness of our findings, and the significant association we found between prenatal exercise and IGWG should be interpreted with caution. IGWG in these women may be the result of an imbalance between caloric intake and energy expenditure, or medical management. Given that women who gain below GWG recommendations have increased odds of SGA and preterm birth,2 prenatal care providers should follow weight gain throughout pregnancy to ensure that women who are exercising during pregnancy gain appropriately, within the IOM guidelines. Counselling about adequate nutrition and increasing awareness about the risk of IGWG is important.
Finally, although based on a limited number of studies, there was evidence that exercise-only interventions were associated with a reduction in PPWR, measured as early as 2 weeks post-delivery up until 1 year postpartum. Whether these findings are due to the benefit of prenatal exercise on reducing the odds of EGWG or to the long-lasting effect of prenatal exercise on body weight regulation during the postpartum period is unknown, since women who exercise during pregnancy are more likely to resume exercise in the postpartum period.99
There are several strengths of the current systematic review. Rigorous methodological standards (GRADE) were used to guide the systematic review process. This included searching 13 databases for peer-reviewed literature as well as examining grey literature. The inclusion criteria were comprehensive, allowing inclusion of studies of all designs and published in three different languages. Data were combined by meta-analysis. Where it was not possible to include reported results in a meta-analysis, the results were still included and were synthesised narratively. Twenty-six countries from four continents were represented in the included studies. Subgroup analyses allowed identification of possible sources of heterogeneity in the effect of exercise-only interventions on total GWG; however, heterogeneity was still high within subgroups. A limitation of our study is that we were unable to examine the effect of different types of exercise such as resistance training alone on GWG, because there were few distinct types of exercise examined (only aerobic exercise or mixed exercise were examined). This highlights the need for future studies examining the effect of different types of exercise (eg, aerobic vs resistance training) on GWG, as well as the efficacy of exercise interventions initiated during pregnancy to prevent EGWG and continued during postpartum to prevent weight retention, with a focus on women with overweight or obesity. More attention should be paid to monitoring and adequately reporting compliance with the exercise interventions as well as considering factors that may influence compliance and retention of study participants. Finally, no studies looked at the effect of exercising in different trimesters on the odds of EGWG. Given that the timing of EGWG has been reported to differentially affect fetal growth,100 101 future studies addressing this question are needed to establish the optimal time point to start an exercise intervention. Finally, we were unable to identify evidence-based cut-off points for clinically meaningful changes in study outcomes. Accordingly, it is possible that the results may have over- or underestimated the relevance of the findings.
Conclusion
Exercise-only interventions were effective at lowering total GWG and PPWR, and reducing the odds of EGWG while increasing the odds of AGWG. However, the overall magnitude of GWG and PPWR difference between the intervention and control groups was quite small (1.0 kg) and the clinical relevance is unclear. We also found higher odds of IGWG with prenatal exercise, but this result needs to be considered with caution. To achieve at least a 25% reduction in the odds of EGWG, pregnant women needed to accumulate at least 456 MET-min/week of moderate-intensity exercise.
What is already known?
Excessive (EGWG) and inadequate (IGWG) gestational weight gain (GWG) are associated with short- and long-term health issues for mother and child; promotion of adequate GWG (AGWG) is therefore of paramount importance.
Prenatal exercise has been identified as an effective strategy to promote AGWG. However, a dose–response relationship between prenatal exercise and GWG is yet to be established.
The long-term impact of prenatal exercise on body weight regulation during the postpartum period also needs to be clarified.
What are the new findings?
Exercise-only interventions reduced GWG and PPWR by approximately 1.0 kg and decreased the odds of EGWG by 32%.
To achieve at least a 25% reduction in the odds of EGWG, pregnant women need to accumulate at least 456 MET-min/week of moderate-intensity exercise (eg, 105 min/week of brisk walking, water aerobics, stationary cycling or resistance training).
Our results suggest that a 25% increased risk of IGWG would be attained when exercise frequency exceeds four times/week, exercise duration exceeds 40 min or exercise volume exceeds 550 MET-min/week. However, this result should be interpreted with caution because it was based on a limited number of studies (five RCTs).
Acknowledgments
We would like to thank Bailey Shandro (UAlberta), Anne Courbalay (UQTR) and Meghan Sebastianski (Alberta SPOR SUPPORT Unit Knowledge Translation Platform), University of Alberta for their assistance with the meta-analysis. The authors wish to acknowledge Mary Duggan from the Canadian Society for Exercise Physiology who is one of the primary knowledge users.
References
Footnotes
Contributors MHD, S-MR, MFM, GAD, KBA contributed to the conception of the study. MHD, S-MR, MFM, GAD, KBA, AJG, NB, VJP, CEG, LGS, RB contributed to the design of the study and development of the search strategy. LGS conducted the systematic search. FS, MJ, VM, RS, LR, MN, TSN, AW, AJK, AAM completed the acquisition of data. MHD, NB, MN performed the data analysis. All authors assisted with the interpretation. S-MR and MHD were the principal writers of the manuscript. All authors contributed to the drafting and revision of the final article. All authors approved the final submitted version of the manuscript.
Funding This project was funded by a Canadian Institute of Health Research Knowledge Synthesis Grant. MHD is funded by an Advancing Women’s Heart Health Initiative New Investigator Award supported by Health Canada and the Heart and Stroke Foundation of Canada. A-AM is funded by a Fonds de Recherche du Québec–Santé Doctoral Research Award. RS is funded by a Canadian Institutes for Health Research Doctoral Research Award.
Competing interests None declared.
Patient consent Not required.
Provenance and peer review Not commissioned; externally peer reviewed.