Background: Both teenage and delayed childbearing were associated with adverse pregnancy outcomes. However, the optimal maternal age that is associated with the lowest risk of fetal and infant mortalities and morbidity is unclear. Methods: In this retrospective cohort, we pooled nulliparous singleton births from the U.S. Cohort Linked Birth-Infant Death Data, Natality and Fetal Death Data from 2011 to 2017. The study population included non-obese (body mass index < 30 kg/m2) nulliparous women

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... th singleton gestation and greater than 20 weeks of gestation. The optimal maternal age at first childbirth for the lowest risk of fetal death, infant mortality, and neonatal morbidity (early preterm birth and severe small-for-gestational-age) was identified by using a second-degree fractional polynomial regression, stratified by race/ethnicity.Results: A total of 5,285,126 singleton births were analyzed for the outcomes of infant mortality and morbidity, and 4,386,757 were analyzed for fetal mortality rate. Non-Hispanic White, Hispanics and Asian American women shared similar maternal age patterns. Maternal age at 26-31 years was associated with the lowest risk of adverse fetal and infant outcomes. However, non-Hispanic Black had the optimal age 2-4 years earlier than other race/ethnic groups, namely 24-27 years old.Conclusions: Maternal age for the optimal pregnancy outcomes is around 24 to 31 years old for the majority of first-time mothers. Our results may provide useful information for counseling women regarding optimal timing of childbearing.