Approximately 90% of pregnant women develop a degree of unilateral or bilateral dilatation of the renal pelvis and ureter by the third trimester. It usually begins at the end of the first trimester and increases as the pregnancy advances. Right-sided dilatation predominates in most patients and the ureters below the pelvic brim are spared. The main cause is partial obstruction by the enlarging uterus compressing the ureters against the iliac arteries as they enter the pelvis. Maternal hormones, which decrease ureteric tone, appear to play a relatively minor role. In most women, dilatation disappears postpartum, sometimes in a few days, but usually more gradually over several weeks.
Renal calculi need to be excluded in pregnant women with loin pain, haematuria, or infection of the urinary tract. Abdominal radiographs are of limited value because the gravid uterus and the fetal skeleton can obscure ureteric calculi.
US demonstrates the presence of calyceal dilatation but cannot differentiate dilatation caused by calculi from dilatation due to pregnancy.
MR urography is now widely available and can provide good anatomical images of the dilated urinary tract without the use of contrast agents or radiation. It should be the investigation of choice in pregnant patients with loin pain. A regular tapering appearance can be seen in extrinsic compression due to the gravid uterus while stones are visible as filling defects with a signal void at the level of the obstruction.
IVU can be performed if US is equivocal and MRI is unavailable but the number of images should be kept to a minimum. Although an image obtained 1h after injection of contrast medium is appropriate if the dilatation is mild on US examination, the interval should be increased to 4–6 hours in patients with marked dilatation.
CT should not be used to evaluate stone disease in pregnancy because of the high dose of radiation.