Objective: To clarify the optimal estimated date of delivery for multifetal pregnancies.
Design, subjects, and setting: A retrospective study of all 88,936 infants born of multifetal pregnancies and all 6,020,542 infants born of singleton pregnancies that occurred at 26 weeks or more of gestation between 1989 and 1993 in Japan.
Main outcome measure: Incidence of stillbirth and early neonatal death according to gestational age at delivery.
Results: The mean +/- SD duration of pregnancy was 37.0 +/- 2.7 weeks for multifetal pregnancies and 39.6 +/- 1.6 weeks for singleton pregnancies. In multifetal pregnancies, the incidence of stillbirth and of early neonatal death gradually declined until 37 to 38 weeks' gestation and then increased. These parameters in singleton pregnancies declined until 39 weeks' gestation before increasing. The lowest incidence of perinatal death (Stillbirth plus early neonatal death) seen at 38 weeks' gestation in multifetal pregnancies corresponded to that seen at 43 weeks' gestation in singleton pregnancies (10.5 vs 9.7 per 1000 infants). The fist of perinatal death was more than 6 times as high for fetuses of multifetal pregnancies born at 37 weeks or later than for singleton fetuses born at 40 weeks or later (relative risk, 6.6; 95% confidence internal, 6.1 - 7.1).
Conclusion: Fetuses of multifetal pregnancies are at an increased risk of death after reaching the normative gestational age for singleton pregnancies. Limiting the estimated date of delivery to 37 to 38 weeks may be appropriate in multifetal pregnancies.