Antenatal renal pelvic measurements: what do they mean?

To analyse data on cases with urological abnormalities recorded over a 15-year period (1984-98) by the Northern Region Congenital Abnormality Survey (NorCAS), with special reference to fetal renal pelvic measurements.

Data on all urological cases recorded in the NorCAS database and in notification documents forwarded from hospitals in the Northern Region were extracted and incorporated in a separate unattributable database for analysis. Specific fields for filing recorded fetal renal pelvic measurements with their corresponding gestational ages were inserted; it was ensured as far as possible that these measurements were made in the anteroposterior plane.

The total number of cases analysed was 2737; a measurement of the fetal renal pelvis was recorded in 813, and more than once in 468, giving 1301 measurement episodes with the corresponding gestational ages, pregnancy and postnatal outcome. The male/female (M/F) ratio was significantly higher in those cases which had a fetal renal pelvic measurement recorded than in those which had not. This was because there was a 12/1 M/F ratio in measured cases of vesico-ureteric reflux (VUR). Few cases of VUR had a fetal renal pelvic measurement recorded. The difference in the fetal renal pelvic measurements between those cases which were normal postnatally and those which had a structural abnormality, an obstructive lesion or VUR became significant at 7 mm and at 18 weeks of gestational age. Over the whole period the proportion of cases in which micturating cysto-urethrography was technically possible and might have contributed to the diagnosis was 28.5%; the highest was 44.1% in 1998.

Antenatal renal dilatation occurs infrequently in the presence of VUR and when it does, the fetus will probably be male. Antenatal ultrasonography is unlikely to contribute to detecting VUR in females, the gender in whom early diagnosis is particularly desirable. Other means of suspecting VUR antenatally, e.g. the family history or genetic coding, may be more valuable. A measurement of the fetal renal pelvis of > or = 7 mm at a gestational age of 18 weeks should prompt subsequent careful ultrasonography during the pregnancy and early postnatal investigation of the urinary tract. The diagnosis of VUR should not be excluded because the fetal renal pelvis is only minimally dilated if the fetus is female.