In order to study the pathogenesis of prenatal deformities, we reviewed maternal histories, delivery records, pathology reports, radiographs, and photographs of 90 fetuses with prenatally documented oligohydramnios at gestational ages from 14 weeks to term. The causes of oligohydramnios included premature rupture of membranes (44 cases), fetal renal insufficiency (25 cases), idiopathic (15 cases), and twin-twin transfusion (6 cases). The fetuses were grouped according to gestational age at delivery and duration of oligohydramnios. Sixty-three fetuses (70%) had documented contractures. As expected, contractures were more frequent with earlier onset and longer duration of oligohydramnios. During the 2nd trimester, the frequency of contractures in fetuses with oligohydramnios was 77% compared to 52% in the 3rd trimester ( 2 = 5.33, 1 df, P = .02). Considering all gestational ages together, 57% of fetuses had contractures after less than 2 weeks of oligohydramnios compared to 81% of fetuses with a longer duration of oligohydramnios (( 2 = 6.23, 1 df, P < .02). The type of contracture varied with gestational age. Clubfoot was the most frequent at all ages, but hand contractures such as camptodactyly were common only in the 2nd trimester while the broad flat hand originally described in Potter sequence was found almost exclusively in the fetuses with oligohydramnios in the 3rd trimester. Of the 63 fetuses with oligohydramnios and contractures, 25 (40%) had either additional malformations or family history that could explain contractures independent of oligohydramnios. Am. J. Med. Genet. 86:430-433, 1999.