Our literature search produced 442 citations, of which we selected 18 for further review of the full text (). Ten studies were excluded for reasons listed in . Therefore, eight publications met our criteria for inclusion in this meta-analysis and review (22
) (). Findings for the OMEGA Study prospective cohort and Alpha Study case-control population were presented in three different publications (26
). We assessed which outcomes were reported more than once to avoid inclusion of duplicate effect estimates in our meta-analyses. Two publications by Dempsey et al. (27
) reported results for the OMEGA Study and Alpha Study populations, including both prepregnancy and early pregnancy exercise exposures. In a more recent, single publication, Rudra et al. (26
) updated the results for both study populations, but for the prepregnancy exposure only. Therefore, the publications by Dempsey et al. (27
) were included in our meta-analysis for their early pregnancy results only, and the relative risks from Rudra et al. (26
) were included for its prepregnancy results.
Study attrition diagram. PA, physical activity.
Ultimately, the eight studies in our analysis (prepregnancy k
= 7; early pregnancy k
= 5) represented a total of 34,929 subjects (prepregnancy N
= 34,929; early pregnancy N
= 4,401), with 2,855 total cases of GDM (prepregnancy n
= 2,813; early pregnancy n
= 361) (22
). These included five prospective cohort studies (22
), two retrospective case-control studies (26
), and two cross-sectional surveys (23
). (The total number of study designs is greater than the total number of publications because Rudra et al. [26
] presented results for two distinct studies in the same study.) All studies were conducted among U.S. women except one, which was conducted by Harizopoulou et al. (23
) among Greek participants. In the prospective cohort studies (22
), physical activity interviews or questionnaires for both prepregnancy and early pregnancy habits were administered before participants received their diagnosis of GDM. For the retrospective case-control and cross-sectional studies (23
), participants were asked about their physical activity during their postpartum hospital stay, with the exception of the study by Redden et al. (29
), which collected exposure data 2–7 months postpartum. The prepregnancy time period was defined in six studies as 1 year before the index pregnancy (23
), in one study as 3 months before the index pregnancy (29
), and in one study as the average exposure over several years of follow-up before the index pregnancy (22
). All but one study (29
) reported use of a validated physical activity questionnaire to assess exposure, although only one of these questionnaires was specifically validated in pregnant women, with satisfactory results (24
). GDM was physician-diagnosed in all but one study, which used validated self-report of having received a physician's diagnosis (22
). Other relevant study characteristics are tabulated in .
Units of physical activity varied and included frequency (hours per week), energy expenditure (MET-hours per week), and level of exertion or intensity. Physical activity types included total physical activity as well as specific activities (walking, climbing stairs, and others). In the meta-analyses of total physical activity, five of the eight studies analyzed physical activity in units of energy expenditure (22
), which incorporates both frequency and intensity, whereas three of the eight studies analyzed physical activity in units of frequency only (24
). All but the two cross-sectional studies reported relative risks across quantiles of exposure (23
Total physical activity
Seven studies reported the association between total prepregnancy physical activity and GDM (22
). A meta-analysis of relative risks indicated a 55% lower risk of GDM for women in the highest physical activity quantiles compared with those in the lowest (pooled OR 0.45 [95% CI 0.28–0.75]; P
= 0.002) (). The Cochrane Q statistic indicated significant heterogeneity in study results (Q = 32.6, P
< 0.001), with an I2
value estimating that 82% (63–91%) of the variance is due to between-study differences.
Results of meta-analyses. A: Prepregnancy physical activity. B: Early pregnancy physical activity.
We conducted additional sensitivity analyses to evaluate potential sources of heterogeneity in the results. Meta-regression did not show a significant difference in effect estimates among studies with a prospective versus retrospective study design (meta-regression P
= 0.54) (supplementary Fig. 1, available in an online appendix at http://care.diabetesjournals.org/cgi/content/full/dc10-1368/DC1
). Likewise, meta-regression results indicated a lack of effect measure modification by GDM diagnosis criteria (P
= 0.58), physical activity analysis by energy expenditure versus frequency (P
= 0.40), study size being >100 cases (P
= 0.15), and country of study (P
= 0.078). When we ran the meta-regression on the total number of exposure categories, there was a borderline significant association (P
= 0.053); however, the number of studies in each strata was few. When we stratified by whether studies adjusted for specific confounders, we did not find a statistically significant difference among effect estimates that controlled for family history of diabetes (P
= 0.97), smoking status (P
= 0.23), race or ethnicity (P
= 0.33), parity (P
= 0.67), or socioeconomic status covariables (P
= 0.47). Finally, to evaluate the robustness of the main pooled effect estimate, we removed the largest study by Zhang et al. (22
), which accounted for 62% of the total study participants. This did not substantially alter the pooled OR or significance level (pooled OR 0.39 [0.20–0.73]; P
Five studies reported effect estimates for the association between early pregnancy physical activity and development of GDM (23
). Results for activity during this time period indicated a significant 24% lower risk of GDM associated with the highest activity group compared with the lowest activity group, as shown in (OR 0.76 [0.70–0.83]; P
< 0.0001). The Q test was not significant for heterogeneity but was possibly underpowered because of few studies (Q = 1.83; P
= 0.77). Despite a point estimate of 0%, the I2
statistic suggested that heterogeneity was possible, given the wide CI (95% CI 0–79%). In a sensitivity analysis we removed the study by Harizopoulou et al. (23
) because it contributed to 96% of the weight in the pooled OR. The pooled OR remained statistically significant with a similar magnitude of effect (0.65 [0.43–0.98]; P
Finally, the Egger and Begg tests for the primary analyses did not indicate the presence of publication bias in the analysis of total physical activity (prepregnancy P = 0.30; early pregnancy P = 0.81). Visual inspection of the funnel plot was in agreement with the statistical test, with no apparent asymmetry.
The association between walking and GDM risk was evaluated in three studies (22
). Two studies analyzed the association between walking duration and GDM risk (Oken et al.: >2 h/day vs. ≤2 h/day; Dempsey et al.: >3 miles/day vs. <1 mile/day) (25
). Overall there did not seem to be an association between walking duration and GDM risk (prepregnancy: pooled OR 0.95 [95% CI 0.50–1.83]; early pregnancy: 0.77 [0.51–1.16]). However, when the joint effect of walking duration and usual walking pace was analyzed, there was an inverse association in the prepregnancy time period. In the studies by Dempsey et al. (27
) and Zhang et al. (22
), women who reported a brisk usual walking pace and walked for a longer duration (Dempsey et al.: >2 miles/day; Zhang et al.: >30 min/day) were associated with a lower risk of GDM, compared with women reporting a casual usual walking pace and shorter duration (pooled OR 0.59 [95% CI 0.30–0.87]). This association was slightly attenuated in early pregnancy, as reported by Dempsey et al. but did not reach statistical significance (OR 0.83 [95% CI 0.48–1.45]). Although only three studies reported associations between walking and GDM risk, findings were consistent for an inverse association with intensity of walking pace, although it is unclear whether walking duration (distance or time) has similar benefits.
Two studies assessed the association between stair climbing and GDM risk as the number of flights of stairs climbed per day during the prepregnancy period (22
). They each found a significant inverse association between GDM and the highest category of stair climbing (Dempsey et al.: ≥10 flights/day; Zhang et al.: ≥15 flights/day) compared with women who did not climb stairs, after adjustment for several potential confounders, including prepregnancy BMI (Dempsey et al.: OR 0.47 [95% CI 0.26–0.93]; Zhang et al.: 0.50 [0.27–0.90]; pooled OR 0.49 [95% CI 0.26–0.72]). Dempsey et al. (27
) also assessed stair climbing in early pregnancy and found a similar inverse association (OR 0.26 [95% CI 0.13–0.52]).
Four studies evaluated physical activity of vigorous intensity (22
). Overall, there was an inverse association between participation in vigorous activity compared with no vigorous activity in prepregnancy (pooled OR 0.47 [95% CI 0.19–0.75]). Two studies also reported an association of GDM and vigorous activity intensity in early pregnancy (25
). The pooled effect estimate suggests an inverse association with vigorous physical activity (0.55 [0.21–1.43]), although this did not reach statistical significance.
Few studies addressed the association of sedentary or inactive lifestyle in prepregnancy or early pregnancy with the risk of GDM. In the prospective cohort by Oken et al. (25
), those who reported being sedentary (≤2 h/week total physical activity) has a nonsignificantly higher risk of GDM for both time periods (prepregnancy: OR 1.4 [95% CI 0.7–3.0]; early pregnancy: 1.4 [0.8–2.6]). Hours spent watching television was not associated with GDM risk in two prospective cohort studies (Oken et al.: relative risk 1.03 [95% CI 0.6–1.8]; Zhang et al.: not reported) (22