Objective: To detect associated anomalies, karyotypes and perinatal prognosis of fetuses with single umbilical artery. Method: Fifteen fetuses who have single umbilical artery were evaluated in the obstetrical ultrasonography and medical genetics departments of Osmangazi University. Results: Fifteen fetuses with single umbilical artery were detected during antenatal ultrasonographic examinations. Associated sonographic abnormalities include oligohydramnios (two), intrauterine growth retardation (one), renal agenesis (one), fetal ascites (one), diaphragmatic hernia (one), hydrocephalus (two), and meningomyelocele (one). Complications related to the pregnancy were pre-eclampsia in one case and abruptio placenta in another. Karyotype analysis was available in 11 cases and the only cytogenetic abnormality detected was trisomy 18 in one case. Two cases with hydrocephalus and single umbilical artery were delivered by cesarean section at 34 and 38 weeks, but both died (on the first and fifth days after birth). Five pregnancies were terminated because of intrauterine death (one), severe pre-eclampsia (one), cytogenetic abnormality (one), and multiple congenital anomalies associated with single umbilical artery (two) at 36, 27, 22, 26 and 29 weeks, respectively. Eight of the neonates who had no additional congenital or cytogenetic abnormality were completely normal at birth and during the neonatal period. Diagnoses were confirmed pathologically in all cases. Conclusion: Scanning the umbilical cord should be one of the essential parts of ultrasonographic examination. When single umbilical artery is detected, a detailed ultrasonographic examination is necessary to rule out associated abnormalities. We advise fetal karyotyping even when no additional pathology can be detected on ultrasonographic examination. (C) 1997 International Federation of Gynecology and Obstetrics.