Urinary tract infections are the most common bacterial infection during pregnancy, affecting roughly 18% of pregnant women in large U.S. studies. That’s significantly higher than the rate in non-pregnant women of the same age, and it comes down to a combination of hormonal shifts, physical changes to the urinary tract, and changes in the urine itself that create near-ideal conditions for bacterial growth.
Hormones Relax the Urinary Tract
Progesterone rises steadily throughout pregnancy, and one of its effects is relaxing smooth muscle tissue everywhere in the body, including the tubes (ureters) that carry urine from the kidneys to the bladder. When those tubes lose muscle tone, they widen and slow down, meaning urine doesn’t flow as briskly as it normally would. This pooling of urine, called urinary stasis, gives bacteria more time to multiply before being flushed out.
The result is a condition called hydronephrosis, where the kidneys and ureters become mildly swollen with retained urine. Asymptomatic hydronephrosis occurs in over 90% of pregnant women, with symptoms peaking between 24 and 28 weeks. It’s considered a normal physiological change, but it directly increases infection risk by letting bacteria sit in the urinary tract longer than they otherwise would.
The Growing Uterus Presses on the Bladder
As the uterus expands, it physically compresses the bladder and can press against the urethra. This makes it harder to fully empty the bladder when you urinate. The leftover urine becomes a breeding ground for bacteria, particularly E. coli and other bacteria that naturally live in the gastrointestinal and genital tracts. In some cases, a retroverted (backward-tilting) uterus can displace the cervix and partially obstruct the urethra, making incomplete emptying even worse.
This mechanical pressure combines with the hormonal relaxation to create what’s called vesicoureteral reflux, where urine flows backward from the bladder toward the kidneys. That backflow can carry bacteria upward into the kidneys, which is one reason kidney infections are more common during pregnancy than at other times.
Urine Chemistry Changes
Pregnancy also alters the composition of urine in ways that favor bacterial growth. The pH of urine rises, making it less acidic. At the same time, the kidneys filter more glucose into the urine, a mild form of glycosuria that’s common even in women without diabetes. Bacteria, especially E. coli, thrive in this warmer, sweeter, less acidic environment. These chemical shifts are subtle enough that you wouldn’t notice them, but they meaningfully change how hospitable your urinary tract is to infection.
Why E. Coli Is Usually the Culprit
UTIs in pregnancy are caused by the same bacteria responsible for UTIs in non-pregnant women. E. coli accounts for the vast majority of cases, followed by other gram-negative bacteria. These organisms live naturally in the lower GI tract and can migrate to the urethra, which is anatomically close in women. The short female urethra already makes this migration easy. During pregnancy, the combination of urine stasis, incomplete bladder emptying, and glucose-rich urine means that bacteria which make it into the bladder are far more likely to establish an infection rather than being flushed out.
The Risks of Untreated Infections
UTIs during pregnancy aren’t just uncomfortable. They carry real risks for both the pregnant person and the baby. Infection often starts as asymptomatic bacteriuria, bacteria in the urine with no symptoms. Left untreated, it progresses to a bladder infection (cystitis) in about 30% of cases and can reach the kidneys (pyelonephritis) in up to 50% of untreated patients. That progression rate is dramatically higher than in non-pregnant individuals.
Kidney infections during pregnancy are associated with preterm birth. In a large California study, 14.9% of women who developed pyelonephritis in the first 36 weeks delivered preterm, compared with 8.1% of women without a UTI. Even lower-tract infections carried an elevated risk of early delivery, with adjusted risk ratios between 1.1 and 1.4 for births before 32 weeks, between 32 and 36 weeks, and between 37 and 38 weeks. Notably, the risk of preterm birth remained elevated even when UTIs were treated with antibiotics, though treatment still reduced the chance of progression to kidney infection.
Why Screening Happens Early
Because asymptomatic bacteriuria is so common in pregnancy (2% to 10% of pregnant women) and so likely to progress to a symptomatic infection, the American College of Obstetricians and Gynecologists recommends screening every pregnant person with a urine culture early in prenatal care. This is a routine part of your first prenatal visit. If the culture comes back positive with significant bacterial counts, your provider will prescribe a short course of antibiotics, typically 5 to 7 days, even though you have no symptoms. This proactive approach is unique to pregnancy; in non-pregnant adults, asymptomatic bacteriuria is usually left alone.
Treatment During Pregnancy
About 20% to 25% of pregnant women receive antibiotics at some point during pregnancy, and UTIs are the leading reason. Several antibiotic classes are considered safe, including penicillins (like amoxicillin and ampicillin), cephalosporins, and fosfomycin. Nitrofurantoin is also commonly used, particularly in the first trimester.
Certain antibiotics are avoided. Fluoroquinolones (like ciprofloxacin) are off the table due to potential kidney, heart, and nervous system effects on the fetus. Tetracyclines are avoided because of proven harm to fetal development. Sulfa-based antibiotics carry enough risk that they’re only used when no better option exists. Your provider will choose a targeted antibiotic based on the specific bacteria found in your urine culture, which is one reason a culture is preferred over a simple dipstick test during pregnancy.
When Infections Peak by Trimester
UTI risk isn’t evenly distributed across pregnancy. The anatomical changes that promote infection, particularly ureteral dilation and bladder compression, become more pronounced as pregnancy progresses. Hydronephrosis peaks between 24 and 28 weeks, placing the late second and early third trimesters at the highest risk. However, infections can occur at any point, including the first trimester, which is why early screening matters.
Recurrent UTIs are also common. In a study of nearly 42,000 women across 10 U.S. sites, 18% reported at least one UTI during pregnancy, with some women experiencing infections in multiple trimesters. If you’ve had one UTI during pregnancy, staying alert to symptoms like painful urination, urgency, lower abdominal pressure, or cloudy urine is especially important since a second infection is more likely than average.