English: Distribution of gestational age at childbirth among singleton live births in the United States, given both when gestational age is estimated by first trimester ultrasound and directly by last menstrual period. About 80% of childbirths occur between 37 and 41 weeks of gestational age, with a somewhat more narrow span when based on first trimester ultrasound. Adapted from data of the following study:

• (2008). "Comparison of gestational age at birth based on last menstrual period and ultrasound during the first trimester". Paediatric and Perinatal Epidemiology 22 (6): 587–596. DOI:10.1111/j.1365-3016.2008.00965.x. ISSN 02695022.