Improved prediction of gestational age from fetal head measurements.

Gestational age estimation based on biparietal diameter alone was compared with three approaches to age prediction that consider head shape: area-corrected biparietal diameter, circumference-corrected biparietal diameter, and head circumference. The analysis was based on 67 fetuses (13-41 weeks) in whom age was known with a high degree of accuracy, using a crown-rump length measurement from a prior first trimester sonographic examination. The three methods that consider head shape are more precise than biparietal diameter alone (p less than 0.05 for second trimester cases, less significantly demonstrated for third trimester cases). While the three appear to be equal to one another in accuracy, area correction may be the preferred approach based on theoretical grounds and on convenience. It is easy and can be used in conjunction with any formula (or table) now applied to biparietal diameter alone.