536 CHAPTER 14 Urological problems in pregnancy

Hydronephrosis

Hydronephrosis develops from ~week 6 to week 10 of gestation. By week 28 of gestation, 90% of pregnant women have hydronephrosis. It has usually resolved within 2 months of birth.

It is due to a combination of the smooth muscle relaxant effect of progesterone and mechanical obstruction from the enlarging fetus and uterus, which compresses the ureter. (Hydronephrosis does not occur in pelvic kidneys or those transplanted into ileal conduits, nor does it occur in quadripeds such as dogs and cats, where the uterus is dependent and thus falls away from the ureter.)

The hydronephrosis of pregnancy poses diagnostic difficulties in women presenting with flank pain thought to be due to a renal or ureteral stone. To avoid using ionizing radiation in pregnant women, renal ultrasonography is often used as the initial imaging technique in those presenting with flank pain.

In the nonpregnant patient, the presence of hydronephrosis is taken as surrogate evidence of ureteral obstruction.

Because hydronephrosis is a normal finding in the majority of pregnancies, its presence cannot be taken as a sign of a possible ureteral stone. Ultrasound is an unreliable way of diagnosing the presence of stones in pregnant (and in nonpregnant) women.

In a series of pregnant women, ultrasound had a sensitivity of 34% (i.e. it misses 66% of stones) and a specificity of 86% for detecting an abnormality in the presence of a stone (i.e., false-positive rate of 14%).1

1 Stothers L, Lee LM (1992). Renal colic in pregnancy. J Urol 148:1383–1387.