Introduction Isolated antenatal hydronephrosis (ANH), defined as a dilation of the renal pelvis (≥5 mm), is one of the most common abnormalities detected on prenatal ultrasound. However, established cut-off values for postnatal follow-up differ between countries and are based on little evidence. The current protocol in the Netherlands for follow-up might be too conservative. Objective To assess the applicability of a higher threshold for follow-up of isolated antenatal hydronephrosis (ANH) than the current practice, without the risk of missing significant postnatal urinary tract obstruction. Study design Retrospective case series on all fetuses with isolated ANH (anteroposterior pelvic diameter (APPD) ≥5 mm) at the second trimester anomaly scan, and diagnosed between 2000 and 2009, in relationship to renal outcome. Results A total of 279 infants with second trimester isolated ANH were included. In 201/279 (72%) fetuses, ANH had normalized (APPD <10 mm) before the third trimester scans. Hydronephrosis persisted postnatally in a minority of 56/279 (20.1%). Postnatal assessment showed signs of obstruction in 41/279 (14.7%) infants, duplicated collecting system or vesicoureteral reflux (VUR). Surgery was performed in 18/279 (6.5%) infants. A non-functioning kidney was shown in 6/279 (2.2%) infants with ANH. Mild ANH (APPD <7 mm) during the second trimester (172/279 (61.6%)) never resulted in surgery for renal anomalies or non-functioning kidney. Infants with an APPD ≥10 mm in the second trimester were far more likely to develop renal anomalies or undergo surgery compared with infants with an APPD 7–10 mm. The number of non-functioning kidneys was too low to be conclusive. Conclusions Follow-up ultrasounds for isolated ANH ≥7 mm instead of ≥5 mm at the second trimester scan would have saved 62% of third trimester scans, without missing any infant with a non-functioning kidney, significant obstruction or symptomatic VUR. In view of the results, it is reasonable that referral for follow-up of second trimester ANH is not strictly indicated in cases with an APPD <7 mm.
- Anteroposterior pelvic diameter (APPD)