Cephalic index: A gestational age-dependent biometric parameter

Abstract

Cephalic index was proposed originally as a means of recognizing altered head shape and confirming the validity of biparietal diameter (BPD) measurements. Two previous studies found cephalic index to be gestational age-independent. We tested this relationship by regression analysis in 777 well-dated pregnancies, evenly distributed between 14–40 weeks’ gestation. Significant variation in cephalic index was found with advancing gestational age, with the highest and lowest values observed at 14 and 28 weeks, respectively. Based upon application in a separate test population of 1361 normal fetuses, a threshold of ± 1 SD maximized identification of misleading BPDs due to altered head shape. © 1989 by The American College of Obstetricians and Gynecologists.