Relation of antepartum stillbirth to birthweight and gestational age: Prospective cohort study.

Objectives: To investigate the incidence of antepartum stillbirth in relation to the distribution of neonatal/fetal weight for different gestational ages.

Design: Prospective observational cohort study.

Setting: Obstetric ultrasound departments in two UK maternity hospitals.

Population: 168 966 women with singleton pregnancies attending for routine antenatal care.

Methods: We examined the incidence of antepartum stillbirths, within different birthweight and fetal weight percentile subgroups, conditioning for gestational age.

Main Outcome Measures: Incidence of antepartum stillbirth.

Results: The risk of stillbirth progressively increased for lower birthweight. Considering the 25-75th percentile as the reference category, the relative risks for stillbirth at <37 weeks' gestation were 7.6 (95% confidence interval [CI] 5.7-10.2) <1st percentile, 2.6 (95% CI 1.8-3.7) 1 to 10th percentile, 1.4 (95% CI 0.9-2.1) 10 to 25th percentile, 0.8 (95% CI 0.4-1.5) 75 to 90th percentile, 0.8 (95% CI 0.4-1.7) 90 to 99th percentile, 0.9 (95% CI 0.3-2.5) >99th percentile. The respective values for births at ≥37 weeks' gestation were 5.0 (95% CI 2.9-8.9), 2.1 (95% CI 1.4-3.3), 1.4 (95% CI 0.9-2.1), 1.2 (95% CI 0.7-1.8), 1.0 (95% CI 0.6-1.8) and 4.0 (95% CI 1.8-9.3). The incidence of stillbirth in ongoing low-risk singleton pregnancies gradually increases for smaller fetuses at any gestational point. The higher incidence (5.56%) was evident for fetal weight <1st percentile between 24 and 28 weeks' gestation.

Conclusion: Fetal weight and the weight of the stillborn have a continuous association with the incidence of antepartum stillbirth which is affected by gestational age.