The variable effects of oligohydramnios on the biparietal diameter and the cephalic index.

It is accepted that breech presentation may alter the shape of the fetal calvarium and hence impair the reliability of the biparietal diameter in estimating gestational age. Several authors have suggested that when the fetal skull appears to have an abnormal shape, the cephalic index should be calculated (outer-to-outer measurements at the level of the biparietal diameter of the short axis/long axis x 100). If the cephalic index is more than 1 standard deviation from the mean {less than 74, more than 83), the biparietal diameter may be an unreliable predictor of gestational age. 1 The purpose for presenting the following case report is twofold: first, to illustrate the rapid and marked effect that oligohydramnios can have on the shape of the fetal head; and second, to emphasize that the cephalic index could be normal despite an abnormal cranial configuration.