The frequency of subsequent childbearing and the method of subsequent delivery among 149 primigravidas who required instrumental delivery for midcavity arrest of the fetal head in the second stage of labor and 1258 primigravidas who delivered spontaneously were compared. The frequency of subsequent childbearing was similar in the two groups, but operative delivery for cephalopelvic disproportion (CPD) in a second pregnancy was six times greater in the instrumentally delivered group (11.2 versus 2%; P less than .005). Nevertheless, more than 75% of instrumentally delivered primigravidas who delivered heavier infants in their second pregnancy did so spontaneously. It is concluded that relative CPD is not a common factor necessitating midcavity deliveries, even if cases in which peridural anesthesia is used and deliveries for fetal bradycardia are excluded from consideration. This probably accounts for the fact that over 97% of instrumentally delivered infants suffered no birth trauma or birth asphyxia.