Birthweight‐for‐gestational age charts were based on the analysis of 3888 consecutive births in which the gestational age was estimated by measuring the fetal biparietal diameter before the 20th week of gestation. The data showed, in contrast to previous studies, a linear relation between gestational age and birthweight without inflection after term. The regression parameters showed a strongly significant difference between girls and boys. Thus, boys were 1·5% heavier than girls at 190 days gestation and 3·6% heavier at 300 days gestation. The linearity in the birthweight‐for‐gestational age charts could be due to the more reliable gestational age based on early ultrasound.