Background Overweight and obesity in pregnancy is increasing. While there are well-documented maternal and infant risks associated with obesity in pregnancy, most studies in this area have used Body Mass Index (BMI) as a categorical variable. This can underestimate the effects of different BMI.
Objectives To evaluate the effect of maternal BMI on pregnancy outcomes using BMI categories and actual BMI as a continuous variable.
Methods Data from 58,233 singleton pregnancies from 2009-2011 in the South Australian Pregnancy Outcome Unit’s population database were reviewed. BMI was recorded prior to 20 weeks gestation in 42,153 (72.4%). Those with a BMI <18.5 kg/m2 (n = 1,294) were excluded. The remaining 40,859 pregnancies were analysed both using BMI as a continuous variable and also divided into four BMI categories. Logistic regression models with adjustment for age, parity, ethnicity, smoking status and hospital type (where appropriate) were employed to report odds ratios and 95% confidence intervals.
Results Preliminary analyses have shown that increased maternal BMI leads to linear increases in risk of gestational diabetes mellitus, pregnancy induced hypertension, induction of labour, caesarean section (elective and overall), large for gestational age infants and macrosomic infants (all p <0.01), the odds ratios increasing for every unit rise of maternal BMI across the entire BMI range. Furthermore, pre-term birth, post-partum haemorrhage, increased maternal length of stay, birth injuries, special care and intensive care nursery admissions and infant length of stay, were related to increasing BMI when analysed in BMI categories.
Conclusions Several complications of pregnancy are related to increasing BMI, with several being related to unit increases in BMI rather than to changes in BMI category. Even small differences in BMI have effects on pregnancy outcomes.