What Is the Contribution of Maternal BMI to the Risk of Adverse Pregnancy Outcomes?

The associations between increased maternal body mass index (BMI) in pregnancy and adverse pregnancy outcomes for women and their infants are well known. Describing and understanding risk accurately can be problematic for women and clinicians, with a true picture often obscured by common reporting practices such as categorisation of BMI and use of only relative measures of risk.

The aims were to investigate relationships between maternal BMI, parity, and adverse pregnancy outcomes; provide estimates of both relative and absolute risks of adverse pregnancy outcomes; and consider the potential contribution from unmeasured confounding.

Data from the South Australian Perinatal Outcomes unit from 2013 to 2017 inclusive were analysed for associations between BMI categories, BMI as a continuous variable, parity, and adverse pregnancy outcomes.

Data from 65,285 women were included. A 5 kg/m² increase in maternal BMI was associated with an increased adjusted relative risk (aRR) of gestational diabetes between 1.3 and 1.4 times, though the adjusted risk difference (aRD) was just 3%, irrespective of parity. The aRR of pre-eclampsia was 1.13 times and 1.43 times in nulliparous and multiparous women respectively; however, the aRD was less than 0.1% for all women. The absolute risk of pre-eclampsia at a BMI of 20 was 0.7% for nulliparous and 0.2% for parous women, which increased to a risk of 0.9% for nulliparous and 0.4% for parous women.

While risks of most adverse outcomes increase with increased BMI, we cannot rule out the possibility that a substantial amount of the observed effects are due to unmeasured confounding. Use of BMI categories leads to underestimation of the true risk for some women and overestimation for others. Use of only relative measures of risk may create an impression that the risks associated with BMI are more dramatic than the absolute risks would suggest. Additionally, parity substantially modifies the risks of BMI for many outcomes; for example, a woman with parity 1 and a BMI of 30 has about the same risk of pregnancy induced hypertension, and a lower risk of pre-eclampsia, compared to a nulliparous woman of BMI 20. A more nuanced understanding of the risks of BMI is needed.