Data synthesis and analysis
We obtained pooled and adjusted ORs with 95% CI estimates of GDM for
the exposure and control groups using the DerSimonian and Laird
random-effects method. We calculated the absolute risk difference for
GDM between the exposure and control groups in low- (control group: GDM
risk was assumed to be 2.0%), moderate- (10%), and high- (20%)
prevalence groups, using the pooled odds ratios (OR) and 95% confidence
intervals (CIs). This assumption was made based on a previous report and
our clinical expertise 40.
Publication bias was assessed
qualitatively by visual inspection of the funnel plot and quantitatively
by Egger’s test 41. Where asymmetry was
observed in the funnel plot, we investigated the likely source of this
asymmetry using the contour-enhanced funnel plot.
We evaluated between-study heterogeneity visually, using forest plots,
and quantitatively, using I 2 and
τ2 statistics. We used the Cochrane chi-square test to
calculate I 2 and τ2statistics. We performed a pre-specified subgroup analysis based on
types of exposure (preterm birth, LBW, or SGA). In pre-specified
sensitivity analyses, we used crude ORs instead of adjusted ORs and
excluded studies using non-standard definitions of GDM. Some studies
assessed the risk of GDM among women born with a weight
>4000 g (macrosomia); these studies were excluded from
post-hoc sensitivity analysis, as a previous review has shown a U-shaped
association between mother’s birth weight and GDM risk20.
All analyses were performed using STATA 14.2 (StataCorp LP, Texas) and
RevMan 5.4 (Cochrane Collaboration, UK). Two-sided p- values
<0.05 were considered indicative of statistical significance.