We evaluated the significance of severe oligohydramnios, or anhydramnios, in the second and third trimesters, by determining the range of etiologies as well as the differences in fetal and neonatal outcome. All prenatal ultrasound results on pregnancies found to have severe oligohydramnios over a 7.5-year period at 13-42 weeks' gestation were retrospectively collected. Follow-up results were obtained from review of medical records, autopsies and pathology reports. A total of 250 singleton pregnancies met the criteria of having severe oligohydramnios. A bimodal distribution in gestational age at diagnosis was seen, with more cases diagnosed at 13-21 weeks and at 34-42 weeks. Fetal abnormalities were present in 50.7% of those diagnosed with severe oligohydramnios in the second trimester and in 22.1% of those in the third trimester. There were 10.2% and 85.3% survivors when severe oligohydramnios was diagnosed in the second and third trimesters, respectively. The rate of aneuploidy was at least 4.4% for the entire singleton population. A bimodal distribution of pregnancies presenting with severe oligohydramnios represents two different naturally occurring populations in terms of both etiology and prognosis.