Objective The authors wanted to examine the effect of maternal obesity on the risk of stillbirth, applying the main categories of the relevant condition at death (ReCoDe) classification.

Method The cohort consisted of 48 357 consecutive, unselected births from six NHS maternity units during 2006/7, including 328 stillbirths from 24.0 weeks. Maternal body mass index (BMI) was categorised as <20, 20–24.9, 25–29.9, 30–34.9, 35+. The clinical classification of ReCoDe1 was used to group the stillbirths into three main categories: congenital anomalies: 18%, fetal growth restriction (FGR – defined as <10th customised birthweight centile): 43% and a miscellaneous group which included maternal, placental, umbilical cord and intrapartum related conditions: 39%.

Results Mothers with BMI <20 had an overall lower risk of stillbirth (OR 0.5, CI 0.3 to 0.8), while obese mothers had an elevated risk (BMI ≥35: 1.6, 1.1–2.3). Analysis within subgroups showed no significant association between high BMI and stillbirths due to congenital anomalies or miscellaneous causes. However, the association with BMI was significant for stillbirths in the FGR category (table).

Conclusion The increased risk of stillbirth in mothers with high BMI is due to stillbirths with fetal growth restriction. The lower rate of stillbirths in pregnancies with low maternal BMI may be related to the recently demonstrated higher rate of antenatal detection of fetal growth restriction in thin mothers.