This paper combines earlier results on the relation between birthweight and gestational age, and the relation between fetal weight and ultrasound measurements of the fetal biparietal diameter (BPD) and mean abdominal diameter (AD) to investigate whether preterm infants (viewed as a group) are smaller than unborn fetuses of the same gestational age. The birthweight distribution for each sex at 223 and 258 days gestation was derived from the birthweight‐for‐gestational age charts based on 3888 newborn infants. The sex‐specific intrauterine weight distribution was estimated from ultrasound measurement of the fetal BPD and AD performed on randomly selected fetuses of gestational age 223 and 258 days. The birthweights were lower than the intrauterine weights, especially early in pregnancy and for female infants. Thus, the 10th birthweight centile for girls at day 223 corresponds to the 4th centile of the‘true’ intrauterine weight, and the‘true’ intrauterine 10th centile corresponds to the 25th centile birthweight at day 223.