THE HYDRONEPHROSIS OF PREGNANCY
In 90% of pregnancies the kidneys are hydronephrotic and this develops from approximately week 6 to week 10 of gestation. It has usually resolved within 2 months of birth (Peake et al. 1983). The hydronephrosis of pregnancy is due to a combination of the smooth muscle relaxant effect of progesterone and to mechanical obstruction from the enlarging fetus and uterus, which compress the ureter (hydronephrosis is said not to 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; Robert 1976).
The hydronephrosis of pregnancy poses diagnostic difficulties in women presenting with flank pain thought to be due to a renal or ureteric stone. Because of the desire to avoid using ionising 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 ureteric
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obstruction. Because hydronephrosis is a normal finding in the majority of pregancies, its presence cannot be taken as a sign of a possible ureteric 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%) (Stothers and Lee 1992).
PRESENTATION OF STONES IN PREGNANCY
Flank pain is the usual presentation, with or without haematuria (macroscopic or microscopic). Differential diagnoses include placental abruption, appendicitis, and pyelonephritis, to name but a few.
WHAT IMAGING STUDY SHOULD BE USED TO ESTABLISH THE DIAGNOSIS OF A URETERIC STONE IN PREGNANCY?
Exposure of the fetus to ionising radiation can cause fetal malformations, malignancies in later life (leukaemia), and mutagenic effects (damage to genes causing inherited disease in the offspring of the fetus). Fetal radiation doses during various procedures are shown in Table 8.1.