Objective To investigate the association between maternal obesity and the prevalence of fetal and infant death.

Design Cohort study using prospectively collected data matched to a high-quality population-based registry data of fetal and infant death.

Setting Five maternity units in the North of England.

Participants 40 932 singleton pregnancies delivered between 2003 and 2005, excluding 1092 pregnancies associated with a congenital anomaly and/or maternal pre-gestational diabetes, and 9998 pregnancies with missing maternal body mass index (BMI).

Main Outcome Measures Prevalence, among singleton pregnancies, of late miscarriage (20–23 weeks gestation), termination of pregnancy for fetal anomaly (≥20 weeks gestation), stillbirth (≥24 weeks gestation), fetal death (miscarriage or antepartum stillbirth), perinatal mortality, neonatal mortality, post-neonatal mortality, and all infant mortality. Crude and adjusted (for maternal age, gestational age, where appropriate, birth weight, standardised for sex and gestational age, ethnicity, cigarette smoking status, index of multiple deprivation, and sex of infant/fetus) ORs of each outcome among maternal obese (BMI ≥30 kg/m²) compared to maternal recommended BMI (BMI 18.5–24.9 kg/m²).

Results Compared to women of recommended BMI, obese women were at significantly greater risk of a fetal death (adjusted odds ratio (aOR) 2.94 (95% CI 2.02 to 4.27), p<0.001), including late miscarriage (aOR 3.27 (95% CI 1.43 to 7.44), p=0.005) and antepartum stillbirth (aOR 2.86 (95% CI 1.88 to 4.35), p<0.001), perinatal death (aOR 2.54 (95% CI 1.70 to 3.79), p<0.001), including stillbirth (aOR 2.77 (95% CI 1.86 to 4.13), p<0.001) and early neonatal death (aOR 3.00 (95% CI 1.12 to 8.03), p=0.03), and infant death (aOR 2.49 (95% CI 1.34 to 4.62), p=0.004), including neonatal death (aOR 2.57 (95% CI 1.13 to 5.88), p=0.03). There was no significant association between maternal obesity and post-neonatal death (aOR 2.27 (95% CI 0.89 to 5.80), p=0.09). The effect of obesity on risk of stillbirth was greater among small-for-gestational-age fetuses, but less among current smokers. Except for higher rates of pre-eclampsia among stillbirths, no specific cause of death could explain the increased odds of fetal and infant death among the obese.

Conclusion Early-pregnancy obesity is significantly associated with fetal and infant death, independent of the known relationship with congenital anomalies. Further studies are required to investigate the specific mechanisms involved. In the meantime, women should be made aware of these risks and supported to optimise their weight before pregnancy.