To develop a prospective antepartum method of identifying cephalopelvic disproportion by comparing the diameters of fetal head with those of the maternal midpelvis.Transvaginal ultrasound pelvimetry was performed on 190 healthy primigravidas with cephalic presentation at 28-35 weeks of gestation, and the diameters of their fetal heads were meassured within one week prior to delivery. These indices the cephalopelvic indices of diameter, cirumference and area, were calculated and compared.The cephalopelvic index of diameter (CID), defined as the difference between the mean diameter of the midpelvis and the fetal biparetal diaameter (BPD), showed the highest degree of accuracy (77.9%). Eighty three percent of women with CID less than 15.8 needed operative delivery; 76.2% of those with CId more than 15.8 mm underwent vaginal delivery.Transvaginal ultrasound pelvimetry and the CID by and large seems to be able to identify cephalopelvic disproportion before labor and may help obsetricians choose the most appropriate form of delivery in an uncomplicated vertex presentation.