Head circumference as an index of fetal age.

By means of a light pen coupled with a microprocessor, the fetal head circumference of 594 fetuses was measured and the results compared with the biparietal diameter and abdominal circumference. In a statistically significant proportion of cases, the head circumference was more closely related than the biparietal diameter to both the abdominal circumference and the gestational age of the fetus as calculated from the mother's menstrual history. Possible reasons for this finding are discussed. It is concluded that the head circumference is a more accurate index of the age of the fetus and its growth potential than is the biparietal diameter, and that use of the head circumference should, in consequence, replace that of the biparietal diameter in obstetric scanning.