The management of patients with preterm premature rupture of the membranes is often influenced by the ultrasound measurement of biparietal diameter. The cephalic index has been used in pregnancies with intact membranes to predict the presence of an abnormal biparietal diameter that may be unreliable in estimating gestational age. This study analyzed 100 patients with rupture of the membranes between the beginning of gestation week 28 and the end of week 34. The cephalic index was found to be abnormal 45% of the time; all were in the brachiocephalic range. The biparietal diameter was affected by presentation and presence or absence of amniotic fluid (p less than 0.05). Use of the biparietal diameter was found to be unreliable in estimating gestational age with preterm premature rupture of the membranes. Use of the cephalic index did not enable prediction of which biparietal diameters would be abnormal; therefore use of the biparietal diameter or the cephalic index in preterm premature rupture of the membranes is not recommended. The use of the head circumference, abdominal circumference, and femur measurements may be a better way of predicting gestational age in this select population.