Genetically determined body mass index and maternal outcomes of pregnancy: a two-sample Mendelian randomization study

Objective: Observational studies have described associations between obesity and adverse outcomes of pregnancy. Mendelian randomization (MR) takes advantage of the ‘natural’ genetic randomization to risk of an exposure such as body mass index (BMI) to study the effects of the exposure on outcomes. Similar to randomization in a clinical trial, this limits the potential for confounding and bias. Design: A two-sample MR study. Setting: Summary statistics from published genome wide association studies (GWAS) in European ancestry populations. Population or Sample: Instrumental variants for body mass index (BMI) were obtained from a study on 434,794 females. Female-specific genetic association estimates for outcomes were extracted from the sixth round of analysis of the FINNGEN cohort data. Methods: Inverse-variance weighted MR was used to assess the association between BMI and all outcomes. Sensitivity analyses with weighted median and MR-Egger were also performed. Results: A 1-SD increase in BMI was associated with higher risk of pre-eclampsia (OR 1.68, 95%CI 1.46-1.94, p=8.74x10-13), gestational diabetes (OR 1.67, 95%CI 1.46-1.92, p=5.35x10-14), polyhydramnios (OR 1.40, 95%CI 1.00-1.96, p=0.049). There was evidence suggestive of a potential association with higher risk of premature rupture of membranes (OR 1.16, 95%CI 1.00-1.36, p=0.050) and postpartum depression (OR 1.12, 95%CI 0.99-1.27, p=0.062). Conclusions: Higher maternal BMI is associated with marked increase in risk of pre-eclampsia, gestational diabetes and polyhydramnios. The relationship between BMI and premature rupture of membranes and postpartum depression should be assessed in further studies. Our study supports efforts to target BMI as a cardinal risk factor for maternal morbidity.