Postpartum evaluation of fetal hydronephrosis: optimal timing for follow-up sonography.

Although intrauterine hydronephrosis can be detected readily by ultrasound, follow-up ultrasound evaluation on the first day of life may show a falsely normal-appearing kidney. The authors present three infants in whom subsequent studies confirmed the presence of upper urinary tract obstruction. It is postulated that the normal-appearing kidneys are probably due to a relative state of dehydration during the first 24 hours of life and a relatively low glomerular filtration rate. It is recommended that the initial postpartum ultrasound examination be performed several days after delivery in neonates diagnosed as having intrauterine hydronephrosis.