Why UTI Risk is Increased During Pregnancy
The risk of urinary tract infections during pregnancy is primarily increased due to mechanical compression of the ureters by the enlarging uterus, which leads to urinary stasis and creates an environment conducive to bacterial growth. 1
Primary Anatomical Mechanisms
Ureteral Compression and Urinary Stasis
- The growing uterus causes external compression on the ureters, particularly after 20 weeks of gestation, which is the dominant anatomical factor increasing UTI risk 1
- This mechanical compression leads to physiological hydronephrosis that is so common it can be considered a normal finding after week 20 of pregnancy 1
- The resulting urinary stasis creates an ideal environment for bacterial colonization and ascending infection 2
Structural and Functional Changes
- Pregnancy induces physiological, structural, and functional urinary tract changes that specifically promote ascending infections from the urethra 2
- These anatomical alterations facilitate bacterial migration upward through the urinary system 3
Clinical Consequences of These Changes
Progression from Asymptomatic Bacteriuria to Pyelonephritis
- Women with asymptomatic bacteriuria in early pregnancy have a 20-30-fold increased risk of developing pyelonephritis during pregnancy compared to 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
- Even asymptomatic bacteriuria places the mother at risk for low birth weight and preterm birth 4
Maternal and Fetal Complications
- These infections are associated with premature delivery and low birth weight infants, making screening and treatment critical 1
- Pyelonephritis can result in significant maternal and fetal morbidity and mortality 4
- The high risk of ascending infection means even uncomplicated UTIs carry substantial risk for both woman and fetus 5
Important Clinical Pitfalls to Avoid
Misinterpreting Physiological Hydronephrosis
- Hydronephrosis in pregnancy should not always be assumed to represent 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
Screening Implications
- Unlike the general population, all pregnant women should be screened for bacteriuria with urine culture 2
- Asymptomatic bacteriuria must be treated in every case diagnosed, as it is an important risk factor for pyelonephritis in this population 2
- A single screen-and-treat approach with urine culture in the first trimester is recommended 3