The effect of oligohydramnios on detection of fetal anomalies with sonography

The sonographic examination of fetuses is generally thought to be compromised when oligohydramnios is present because of the subjective 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.

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 underwent nontargeted sonography. Sonographic results in pregnancies with oligohydramnios and without oligohydramnios (control population) were compared. Follow-up was obtained from a perinatal database, autopsy reports, and medical records.

We found 345 patients with a history of premature rupture of the membranes (175 with oligohydramnios, 170 without oligohydramnios). Gestational age of fetuses was 16-38 weeks. Major congenital anomalies, including hydronephrosis, ventriculomegaly, intestinal atresias, hydrops, congenital diaphragmatic hernia, skeletal dysplasias, cloacal malformations, and gastroschisis, were revealed on sonography in 13 of 175 pregnancies with oligohydramnios and in 17 of the 170 pregnancies in the control group. Major anomalies missed in the oligohydramnios group included cardiac anomalies, club foot, small ventral hernia, limb reduction defect, and anal atresia. Major anomalies missed in the control group were club foot, anal atresia, and tracheoesophageal fistula. All of the major anomalies missed in both groups were of the type that are known to be difficult to diagnose before birth and that are frequently missed on sonography.

Although oligohydramnios subjectively degrades image resolution, sonography still reveals important fetal anatomic landmarks. Major anomalies can be detected on sonography even when the pregnancy has less than the normal amount of amniotic fluid.