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Ultrasound assessment of gestational length is based on the assumption that fetuses of the same gestational age have equal size at the time of investigation. However, there are detectable sex differences in fetal size by the end of the first trimester. We examined whether ultrasound dating introduces sex differences in risks of adverse perinatal outcomes related to post-term birth.We used the Swedish Medical Birth Register to compare male and female newborns during 1973–1978, when gestational age was based on the last menstrual period, and 1995–2007, when gestational age was based on ultrasound. We included singleton births from 39 to 43 gestational weeks.During the first time period, the newborn male-to-female ratio by gestational age at delivery was constant around 1.0, but in the later time period it consistently increased by gestational age, reaching 1.60 at 43 weeks. In the first time period, post-term females had reduced risk for adverse perinatal outcomes compared with post-term males. After the introduction of ultrasound, post-term females had higher risks of stillbirth (odds ratio = 1.60 [95% confidence interval = 1.11 to 2.30]) and meconium aspiration (1.39 [1.10 to 1.75]), compared with post-term males. One-third of stillbirths among post-term girls today might be due to incorrect calculation of gestational age.Introduction of ultrasound for the estimation of gestational age may be associated with increased risks of adverse perinatal outcomes among females classified as post-term compared with their male counterparts.