Kidney Swelling Seen Before Birth: The Post-Birth Work-Up for UPJ Obstruction

On the pregnancy ultrasound, the front-to-back diameter (AP diameter) of the baby's renal pelvis is measured. In the literature, a measurement of 4 mm or more at 16–27 weeks, or 7 mm or more after 28 weeks, is recorded as hydronephrosis (kidney swelling); larger diameters and widening of the calyces are considered a more advanced grade.

In babies with one-sided widening, normal amniotic fluid, and a normal-looking bladder, the pregnancy monitoring plan and delivery plan generally do not change. Your obstetrician will repeat the ultrasound in the third trimester to watch the course of the widening.

The first ultrasound is done at least 48 hours after birth, most often on days 3–7. Because a newborn produces relatively little urine in the first days, an ultrasound done earlier can make the widening look smaller than it is and can be misleading.

The post-birth ultrasound is graded with the UTD classification: an AP diameter under 10 mm is considered normal; P1 (10–15 mm, central widening only) is low risk, P2 (15 mm or more, or peripheral calyx or ureter widening) is moderate risk, and P3 (thinning of kidney tissue or a bladder abnormality) is high risk.

If the post-birth widening persists, the ureter is normal, and emptying on MAG3 is slow, the picture is consistent with UPJ obstruction. There is no need to panic at this point: the majority of these children are followed without surgery, and the literature reports that only roughly a quarter eventually need pyeloplasty (the repair operation for UPJ obstruction).

• If a newborn has fever, is not feeding, is drowsy, or shows a marked drop in urine output, see a doctor the same day.
• If you were told about severe widening on both sides or a solitary kidney, do not delay the first ultrasound; watch the baby's urine output.
• If you notice blood in the urine or a palpable swelling in the abdomen, report it without waiting for the planned appointment.