In this work on perinatal epidemiology, our aim was to provide estimates of gestational age at birth that could inform assumptions when clinical estimates of gestational age at birth are not available. For this, we reviewed birth certificates from the US in 2019 and 2020. We found that the median and mode gestational age at birth were 39 weeks overall. This was also true in most groups of singletons, regardless of maternal age, smoking in pregnancy, or maternal body mass index.
Gestational age at birth was lower in some groups, and less narrowly defined. These groups were generally defined by pregnancy or neonatal complications or by multifetal pregnancies, such as twins or triplets.
The results of this research provide information that other researchers can use to estimate the duration of pregnancy in database studies when clinical or obstetrical information is lacking.