Objective Maternal obesity is associated with an increased risk of stillbirth. The authors wanted to study the relationship between stillbirth risk and the antenatal detection of intrauterine growth restriction (IUGR) in mothers with high and low body mass index (BMI).

Method The cohort consisted of 48 357 consecutive pregnancies delivered in one of six participating maternity units. Maternal BMI was grouped as <20, 20–24.9, 25–29.9, 30–34.9 and 35+. IUGR was defined as birthweight below the 10th customised centile. Antenatal diagnosis of IUGR by clinical assessment was based on estimating fetal weight <10th centile, slowed growth and/or abnormal Doppler.

Results Stillbirth risk rose incrementally with BMI categories, from 2.8 (BMI <20) to 6.2, 7.6, 8.5 and up to 9.8 for mothers with BMI of 35 or more. Antenatal detection of IUGR was highest for mothers with BMI<20: 33.3%, and dropped with increasing BMI: 20–24.9: 22.2%; 25–29.9: 22%; 30–34.9: 20.7%; 35+: 17.2%. The reverse rates (% of IUGR missed) are plotted in the figure.

Conclusion The increased risk of stillbirth in pregnancies with high maternal BMI is associated with an elevated rate of fetal growth restriction. The lowest stillbirth risk is in low BMI mothers, where antenatal detection rates of IUGR are highest. The correlation between increasing stillbirth risk and undetected IUGR warrants further investigation, and suggests that at least some of these deaths could be avoided by improved antenatal surveillance.