Fetal inner orbital diameter and outer orbital diameter were measured in the occipital transverse and occipital posterior positions in 180 normal pregnancies. The outer orbital diameter was found to be closely related to the biparietal diameter. After a nomogram had been constructed, 463 patients whose fetuses were at risk for various anomalies were studied in two cases, hypotelorism was suspected because the orbital measurements fell below the 95% confidence limits, and this diagnosis was confirmed at the time of delivery. The conclusion is that, in situations in which fetal head position precludes accurate biparietal diameter determinations, the outer orbital diameter can often be obtained and used to estimate that variable and, therefore, gestational age. Furthermore, the nomogram constructed has proved to be a useful tool in diagnosing hypotension in utero. Theoretically, this should also be true of fetal hypertelorism.