THE EFFECT OF OLIGOHYDRAMNIOS ON DETECTION OF FETAL ANOMALIES WITH SONOGRAPHY

OBJECTIVE. The sonographic examination of fetuses is generally thought to be compromised when oligohydramnios is present because of the subj ective impression of less adequate visualization of fetal anatomy. The aim of this study was to evaluate the extent to which oligohydramnios limits our ability to detect major anomalies on sonograms. MATERIALS AND METHODS. Records from the University of California, San Francisco from March 4, 1989 through January 4, 1994, were reviewed to identify all cases of premature rupture of the membranes in patients who then u nderwent nontargeted sonography. Sonographic results in pregnancies wi th oligohydramnios and without oligohydramnios (control population) we re compared. Follow-up was obtained from a perinatal database, autopsy reports, and medical records. RESULTS. We found 345 patients with a h istory of premature rupture of the membranes (175 with oligohydramnios , 170 without oligohydramnios). Gestational age of fetuses was 16-38 w eeks. Major congenital anomalies, including hydronephrosis, ventriculo megaly, intestinal atresias, hydrops, congenital diaphragmatic hernia, skeletal dysplasias, cloacal malformations, and gastroschisis, were r evealed on sonography in 13 of 175 pregnancies with oligohydramnios an d in 17 of the 170 pregnancies in the control group. Major anomalies m issed in the oligohydramnios group included cardiac anomalies, club fo ot, small ventral hernia, limb reduction defect, and anal atresia. Maj or anomalies missed in the control group were club foot, anal atresia, and tracheoesophageal fistula. All of the major anomalies missed in b oth groups were of the type that are known to be difficult to diagnose before birth and that are frequently missed on sonography. CONCLUSION . Although oligohydramnios subjectively degrades image resolution, son ography still reveals important fetal anatomic landmarks. Major anomal ies can be detected on sonography even when the pregnancy has less tha n the normal amount of amniotic fluid.