Distribution of gestational age by maternal and infant characteristics in US birth certificate data: informing gestational age assumptions when clinical estimates are not available

ABSTRACT We aimed to describe the distribution of gestational age at birth (GAB) to inform the estimation of GAB when clinical or obstetric estimates are not available for perinatal epidemiologic research. We estimated GAB (median, mode, mean, standard deviation) and percentage born at each gestational week in groups based on plurality and other variables for live births in CDC’s US birth data. In 2020, 3,617,213 newborns had birth certificates with nonmissing GAB. Among singletons (3,501,693), median and mode GAB were both 39 weeks. Births with lower median GAB were from women with eclampsia (37 weeks) or receiving intensive care (37 weeks); newborns receiving intensive care (37 weeks); infants with birth weight < 2,500 grams (35 weeks), < 1,500 grams (28 weeks), or < 1,000 grams (25 weeks); and newborns not discharged alive (23 weeks). Among twins (112,633), median GAB was 36 weeks (mode, 37 weeks). Additional noteworthy groups were women with 7-8 (median, 35 weeks) or 0-6 prenatal visits (median, 34 weeks) or aged 15-19 years (median, 35 weeks). Some maternal and infant groups had distinct GAB distributions in the US. This information can be useful in estimating GAB when individual-level clinical estimates are not available.