Biparietal diameter of the skull and fetal weight in the second trimester: an allometric relationship.

Ultrasonic cephalometry was performed prior to therapeutic abortion in 323 women during the second trimester of pregnancy. In 101 instances satisfactory paired measurements of intrauterine biparietal diameter and extrauterine fetal weight were obtained and these formed the basis for mathematical analysis. In midpregnancy increase in size of the fetal head and increase in body weight seem to proceed at different rates but the ratio of the rates appears to be constant--an allometric relationship of relative growth. The data were used to calculate a standard curve and a graphic method is proposed for estimating gestational age when reliable menstrual history is not available in the second trimester.