During pregnancy, there is mechanical compression of the ureters by the enlarging uterus, leading to ureteral dilation (hydroureter) and hydronephrosis. This results in urinary stasis, which creates a favorable environment for bacterial growth, significantly increasing the risk of urinary tract infections. Additionally, progesterone-induced smooth muscle relaxation further decreases ureteral peristalsis, compounding urinary stasis. These changes combined make ureteral compression and dilation the primary physiological factors increasing UTI risk in pregnancy.

Reference: Williams Obstetrics, Cunningham, 25th Edition.