Variations in Mortality and Morbidity by Gestational Age among Infants Born at Term

Objective

To examine the risks of infant death and neonatal morbidity by week of gestation at term.

Study design

National U.S. birth cohort study on the basis of singleton live births in 1995-2001 at 37 to 41 completed weeks gestational age (GA), with exclusion of congenital anomalies. Main outcomes included neonatal, postneonatal, and cause-specific infant death; low-Apgar score at 5 minutes; receipt of neonatal mechanical ventilation ≥30 minutes; neonatal seizures; birth injury; and meconium aspiration syndrome. To reduce confounding by indication, we carried out a secondary analysis restricted to low-risk deliveries.

Results

In non-Hispanic white women, the mortality rate decreased with increasing GA from 37 to 39 weeks, remained stable from 39 to 40 weeks, and then (for neonatal death) increased at 41 weeks. Rates of low 5-minute Apgar score and mechanical ventilation showed a U-shaped relation across term GAs, and rates of meconium aspiration syndrome and birth injury rose with increasing GA. Results were similar among infants born to low-risk mothers and in non-Hispanic black women.

Conclusions

Term infants show considerable heterogeneity across gestational age in neonatal and late infant outcomes, even when born to mothers at low risk. Recent trends toward earlier labor induction may have adverse health impacts.

Abbreviations: GA, Gestational age, LMP, Last menstrual period, SIDS, Sudden infant death syndrome, OR, Odds ratio