Morphologic changes in the renal tract in pregnancy.

The morphology of the urinary tract is altered considerably during gestation. Changes occur as early as the first trimester and increase as pregnancy progresses. The edema and hyperemia that occur in the bladder along with its decreased tone predispose the patient to asymptomatic bacteriuria. The bacteria can then be transmitted to the ureters via the vesicoureteral reflux. The increase in urinary volume in the ureteropelvic system enhances the growth of the organisms and hence is thought to be the major cause of the increased incidence of pyelonephritis in pregnancy. The morphologic alterations regress dramatically as pregnancy is terminated and reach prepregnancy status at most by the third month post partum, a fact of which clinicians should be aware when interpreting diagnostic test results.