Risks of stillbirth, neonatal mortality, and severe neonatal morbidity by birthweight centiles associated with expectant management at term

BACKGROUND: Determining the optimal time of birth at term is challenging given the ongoing risks of stillbirth with increasing gestation vs the risks of significant neonatal morbidity at early-term gestations. These risks are more pronounced in small infants.

OBJECTIVE: This study aimed to evaluate the risks of stillbirth, neonatal mortality, and severe neonatal morbidity by comparing expectant management with delivery from 37+0 weeks of gestation.

STUDY DESIGN: This was a retrospective cohort study evaluating women with singleton, nonanomalous pregnancies at 37+0 to 40+6 weeks' gestation in Queensland, Australia, delivered from 2000 to 2018. Rates of stillbirth, neonatal death, and severe neonatal morbidity were calculated for <3rd, 3rd to <10th, 10th to <25th, 25th to <90th, and ≥90th birthweight centiles. The composite risk of mortality with expectant management for an additional week in utero was compared with rates of neonatal mortality and severe neonatal morbidity.

RESULTS: Of 948,895 singleton, term nonanomalous births, 813,077 occurred at 37+0 to 40+6 weeks' gestation. Rates of stillbirth increased with gestational age, with the highest rate observed in infants with birthweight below the third centile: 10.0 per 10,000 (95% confidence interval, 6.2-15.3) at 37+0 to 37+6 weeks, rising to 106.4 per 10,000 (95% confidence interval, 74.6-146.9) at 40+0 to 40+6 weeks' gestation. The rate of neonatal mortality was highest at 37+0 to 37+6 weeks for all birthweight centiles. The composite risk of expectant management rose sharply after 39+0 to 39+6 weeks, and was highest in infants with birthweight below the third centile (125.2/10,000; 95% confidence interval, 118.4-132.3) at 40+0 to 40+6 weeks' gestation. Balancing the risk of expectant management and delivery (neonatal mortality), the optimal timing of delivery for each birthweight centile was evaluated on the basis of relative risk differences. The rate of severe neonatal morbidity sharply decreased in the period between 37+0 to 37+6 and 38+0 to 38+6 weeks, particularly for infants with birthweight below the third centile.

CONCLUSION: Our data suggest that the optimal time of birth is 37+0 to 37+6 weeks for infants with birthweight <3rd centile and 38+0 to 38+6 weeks' gestation for those with birthweight between the 3rd and 10th centile and >90th centile. For all other birthweight centiles, birth from 39+0 weeks is associated with the best outcomes. However, large numbers of planned births are required to prevent a single excess death. The healthcare costs and acceptability to women of potential universal policies of planned birth need to be carefully considered.