Maternal body mass index and post-term birth: a systematic review and meta-analysis

Background: Post-term birth is a preventable cause of perinatal mortality and severe morbidity. This review examined the association between maternal BMI and post-term birth at ≥42 and ≥41 weeks’ gestation.
Methods: Six databases, reference lists and citations were searched May-November 2015. Observational studies published in English since 1990 were included. Linear and nonlinear doseresponsemeta-analyses were conducted using random effects models. Sensitivity analyses assessed robustness of the results. Meta-regression and sub-group meta-analyses explored heterogeneity.Obesity classes were defined as I (30.0-34.9kg/m2), II (35.0-39.9kg/m2), and III (≥40kg/m2; IIIa 40.0-44.9kg/m2, IIIb ≥45.0kg/m2).Results: Searches identified 16,375 results; 39 studies met the inclusion criteria (n=4,143,700 births). A nonlinear association between maternal BMI and births ≥42 weeks was identified, ORs and 95% CIsfor obesity classes I-IIIb were 1.42 (1.27-1.58), 1.55 (1.37-1.75), 1.65 (1.44-1.87) and 1.75 (1.50-2.04) respectively. BMI was linearly associated with births ≥41 weeks: OR 1.13 (95% CI 1.05-1.21) for each 5 unit increase in BMI.
Conclusions: The strength of the association between BMI and post-term birth increases with increasing BMI. Odds are greatest for births ≥42 weeks among class III obesity. Targeted interventionsto prevent the adverse outcomes associated with post-term birth should consider the difference in risk between obesity classes.