To explore the indications of prenatal ultrasound and magnetic resonance imaging (MRI) in diagnosing fetal genitourinary abnormalities.

Two hundred and twenty-seven fetuses with genitourinary abnormalities detected by prenatal ultrasound received MRI examination within 24 hours after ultrasound. The final diagnosis was confirmed by autopsy or clinical follow-up. The results of prenatal ultrasound and MRI were analyzed retrospectively and compared with follow-up results. And the misdiagnosis of prenatal ultrasound in fetal genitourinary abnormalities were also analyzed.

In 227 fetuses with genitourinary abnormalities, there were 70 fetuses with renal cystic lesions, 41 with urinary obstruction, 40 with renal agenesis, 24 with ectopic kidney, 18 with horseshoe kidney, 17 with renal duplication, 8 with kidney neoplasm, 3 with small kidney, 5 with bladder anomaly, and 1 with small penis. The diagnostic accuracy rates of ultrasound and MRI were 85.5% (194/227) and 98.7% (224/227) respectively. There were 33 cases of fetal genitourinary abnormalities misdiagnosed by prenatal ultrasound as follows: failed to detect the location of ureteral stricture or obstruction in 15 cases of urinary obstruction (including 1 ureteral polyp); failed to diagnose definitely in 3 cases of urinary obstruction (including 1 megaureter and 2 cases of hydronephrosis); 3 cases of urinary obstruction misdiagnosed as renal duplication; and the rest 12 misdiagnosed cases including 3 ectopic kidney, 2 renal duplication associated with cystic renal dysplasia, 2 renal agenesis, 2 horseshoe kidney, 1 polycystic kidney associated with severe hydronephrosis, 1 renal duplication and 1 small penis. There were 3 cases of fetal genitourinary abnormalities misdiagnosed by prenatal MRI. Neither prenatal ultrasound nor MRI could show the genitalia anomaly detected by postnatal examine, in all 3 cases of urinary system abnormalities associated with genitalia anomaly.

In fetal urinary system abnormalities, prenatal ultrasound and MRI have high diagnostic accuracy rates. However, ultrasound and MRI are both difficult to show genitalia anomaly. The indications of MRI in detecting fetal genitourinary abnormalities need to be defined to guide a reasonable use of fetal MRI.