Why UTI Risk Increases During Pregnancy
Pregnancy creates anatomical and hormonal changes that predispose women to urinary tract infections through mechanical ureteral compression by the enlarging uterus and hormonal effects that reduce ureteric peristalsis, leading to urinary stasis. 1
Anatomical Changes
Mechanical compression of the ureters by the enlarging uterus is the primary anatomical factor, particularly after 20 weeks of gestation. 1
- The growing uterus causes external compression on the ureter, leading to hydronephrosis that is commonly seen in later stages of pregnancy. 1
- This compression creates urinary stasis, which allows bacteria more time to multiply and ascend the urinary tract. 2, 3
- The physiological hydronephrosis of pregnancy is so common that it can be considered a normal finding after week 20. 1
Hormonal and Physiological Changes
Hormonal changes during pregnancy reduce ureteric peristalsis, further contributing to urinary stasis. 1
- Progesterone and other pregnancy hormones cause smooth muscle relaxation throughout the urinary tract, decreasing the normal propulsive activity that helps clear bacteria. 2, 3
- These profound physiologic changes of the urinary tract during pregnancy contribute to the increased risk for symptomatic urinary tract infection in women with bacteriuria. 3
- The combination of mechanical obstruction and decreased peristalsis creates an environment conducive to bacterial growth and ascending infection from the urethra. 2
Clinical Consequences
The risk of progression from asymptomatic bacteriuria to pyelonephritis is dramatically increased during pregnancy. 1
- Women with asymptomatic bacteriuria in early pregnancy have a 20-30-fold increased risk of developing pyelonephritis during pregnancy compared with women without bacteriuria. 1
- Without treatment, pyelonephritis develops in 20-35% of pregnant women with asymptomatic bacteriuria, compared to only 1-4% when treated. 1
- These infections are associated with premature delivery and low birth weight infants, making screening and treatment critical. 1
Common Pitfalls to Avoid
Do not assume hydronephrosis in pregnancy always represents infection—it is often physiological compression by the uterus. 1
- However, any pregnant woman with hydronephrosis and fever should be evaluated for obstructive uropathy with infection, as this can lead to preterm labor. 1
- All pregnant women should be screened for bacteriuria at the first antenatal visit, as most symptomatic UTIs develop in women with bacteriuria earlier in pregnancy. 3, 4
- The anatomical and hormonal changes are not reversible during pregnancy, making prevention through screening and early treatment essential rather than attempting to reverse the underlying predisposition. 2, 3