Determination of threshold value for follow-up of isolated antenatal hydronephrosis detected in the second trimester

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.