Infant mortality among singletons and twins in the United States during 2 decades: effects of maternal age.

OBJECTIVE: Whereas it is well-established that maternal age has a U-shaped relationship with infant mortality among singletons, effects on the risk of mortality among multiple births remains unexplored. If there is a relationship, then assessment of maternal age may identify pregnancies and births that should be targeted by both clinical and public health interventions. The objective of this study was to characterize the relationship between maternal age and infant mortality in singleton and twin births in the United States.
METHODS: A retrospective cohort study was conducted of all live births (singleton: n = 22 546 718; twin: n = 535 544) in the United States in 1985-1986, 1990-1991, and 1995-1996. Risks and relative risks for infant deaths by maternal age before and after adjustments for birth cohort, gravidity, birth weight, and gestational age were measured.
RESULTS: Maternal age had a U-shaped association with mortality among singletons, with highest rates seen at extremes of age. Among twins, however, there was a steep and inverse relationship between age and mortality, with those born to young mothers experiencing the highest mortality rates. Seven percent of twin births resulted in an infant death for women who were younger than 20 years, 2.7% for those 30 to 34 years, and 2.0% for women 40 to 49 years. Even after adjustments for gravidity, birth weight, and gestational age, these trends persisted. Additional examination by timing of death indicated that this relationship was primarily a function of postneonatal rather than neonatal mortality.
CONCLUSIONS: The highest mortality among twins occurred to women who delivered in their teens and early 20s. The implications of these findings, both from a clinical and a public health perspective, deserve attention.