Maternal pre-pregnancy body mass index and risk of preterm birth: a collaboration using large routine health datasets

Importance Preterm birth (PTB), is a leading cause of child morbidity and mortality. Objective: To examine the associations of maternal pre-pregnant body mass index (BMI) with any PTB, spontaneous (SPTB) and medically indicated PTB (MPTB). Design A meta-analysis of eight population-based datasets. Setting Three UK datasets, two USA datasets, and one each from South Australia, Norway and Denmark, with different characteristics and sources of bias. Participants All pregnancies resulting in a live birth or stillbirth after 24 completed gestational weeks. Exposure Maternal pre-or early pregnancy BMI derived from self-reported or measured weight and height between 12 months pre-pregnancy and 15 weeks gestation. Main Outcome(s) and Measures(s) Any PTB (delivery <37 completed weeks), SPTB and medically indicated PTB. Fractional polynomial multivariable logistic regression was applied to eight datasets from different high-income countries and time periods. The results were combined using a random effects meta-analysis. Results We found non-linear associations between pre-pregnant BMI and all three outcomes, across all datasets. The adjusted risk of any PTB and MPTB was elevated at both low and high BMIs, whereas the risk of SPTB was increased at lower levels of BMI but remained low or increased only slightly with higher BMI. In the meta-analysed data, the lowest risk of any PTB was at a BMI of 24.5 kg/m 2 (95% confidence interval: 23.1, 30.3), with a value of 21.3 kg/m 2 (20.8, 21.9) for MPTB; for SPTB, the risk remained roughly constant above a BMI of around 25-30 kg/m 2 . Conclusions and Relevance Consistency of findings across different populations, despite differences between them in the time period covered, BMI distribution, missing data and control for key confounders, highlight the importance of promoting pre-conception BMI between 21 to 30 kg/m2 to prevent MPTB and SPTB