Maternal BMI and preterm birth: A systematic review of the literature with meta-analysis

Objectives. To examine the association between high prepregnancy maternal body mass index (BMI) and the risk of preterm birth (PTB). Methods. A systematic review of the literature. We included cohorts and case-control studies published since 1968 that examined the association between BMI and PTB of all types, spontaneous (s), elective and with ruptured membranes (PPROM) in three gestational age categories: general (<37 weeks), moderate (32-36 weeks) and very (<32 weeks) PTB. Results. 20,401 citations were screened and 39 studies (1,788,633 women) were included. Preobese (BMI, 25-29.9) and obese I (BMI, 30-34.9) women have a reduced risk for sPTB: AOR - 0.85 (95% CI: 0.80-0.92) and 0.83 (95% CI: 0.75-0.92), respectively. Their risk for moderate PTB was 1.20 (95% CI: 1.04-1.38) and 1.60 (95% CI: 1.32-1.94), respectively. Obese II women (BMI, 35-40) have an increased risk for PTB in general (AOR = 1.33, 95% CI: 1.12-1.57) moderate (AOR - 2.43, 95% CI: 1.46-4.05) and very PTB (AOR - 1.96, 95% CI: 1.66-2.31). Obese III women (BMI > 40) have an even higher risk for very PTB (AOR = 2.27, 95% CI: 1.76-2.94). High BMI does not modify the risk for PPROM and increases the risk for elective PTB. Conclusions. High maternal BMI may have different effects on different types of PTB.