Measurement of the baby’s head (cephalometry) is usually done by ultrasound and it varies with the period of gestation (weeks of pregnancy) and the ethnic group. It increases from about 2.4 cm at 13 weeks to about 9.5 cm at term. Different babies of the same weight can have different head size, therefore dating in the later part of pregnancy is generally considered unreliable. In the 38th week it ranges from 8.8 cm to 10.4 cm. To the best of my knowledge there is no correlation between the size of the baby and the mother’s blood group.