Pregnancy itself is the primary cause of UTIs during pregnancy. The hormonal shifts, anatomical changes, and pressure from a growing uterus all work together to slow urine flow and create conditions where bacteria thrive. Between 2% and 10% of pregnant women develop bacteria in their urine, and roughly 9.4% experience a symptomatic urinary tract infection during their pregnancy. Understanding why this happens can help you recognize the risk early and take steps to prevent it.
How Pregnancy Changes Your Urinary Tract
Your body starts remodeling the urinary system surprisingly early. Progesterone, the hormone that rises sharply to maintain pregnancy, relaxes smooth muscle throughout your body. That includes the ureters, the tubes that carry urine from your kidneys to your bladder. When those tubes relax and lose their normal muscle tone, urine moves more slowly and can pool instead of draining efficiently. This pooling, called urinary stasis, gives bacteria extra time to multiply.
On top of the hormonal effects, the growing uterus physically compresses the bladder and ureters as pregnancy progresses. The pelvic floor begins lowering as early as 12 to 16 weeks, and by the third trimester the weight of the baby pressing on the bladder makes it harder to empty completely. Residual urine left in the bladder after each trip to the bathroom acts as a warm, nutrient-rich reservoir for bacterial growth. The combination of slower flow and incomplete emptying is the core mechanical reason UTIs spike during pregnancy.
Which Bacteria Are Responsible
E. coli causes 70% to 80% of all UTIs in pregnancy. It’s a normal resident of the digestive tract that can migrate to the urethra, especially given the short distance between the anus and the urethral opening. Because pregnancy already slows urine flow, even a small amount of E. coli that reaches the bladder has a better chance of establishing an infection than it would outside of pregnancy.
The remaining cases are split among several other organisms. Klebsiella and Proteus species each account for about 5% of infections. Group B Streptococcus, a bacterium that also matters during labor and delivery, causes 2% to 5% of pregnancy UTIs. Gram-positive bacteria collectively are responsible for up to 10% of cases. Knowing the specific bacterium matters because your provider will choose an antibiotic that targets whichever organism shows up in your urine culture.
Risk Factors That Raise Your Odds
While every pregnant person faces the universal risk factors described above, some women are more susceptible than others. Pre-existing diabetes is one of the strongest individual risk factors, associated with about a 39% higher prevalence of UTIs during pregnancy compared to women without diabetes. Certain urologic conditions, such as a history of kidney stones or structural abnormalities in the urinary tract, also increase risk.
A personal history of recurrent UTIs before pregnancy is another red flag. If your body was already prone to infections, the added hormonal and mechanical changes of pregnancy compound that vulnerability. Sexual activity can introduce bacteria into the urethra, and frequency of intercourse during pregnancy has been linked to higher UTI rates as well.
Why Symptom-Free Infections Still Matter
One of the more important things to understand about UTIs in pregnancy is that you can have a significant bacterial infection in your urine and feel completely fine. This condition, called asymptomatic bacteriuria, is found in 2% to 10% of pregnant patients through routine urine culture screening. It produces no burning, no urgency, no obvious signs.
Left untreated, though, asymptomatic bacteriuria progresses to a kidney infection (pyelonephritis) in 20% to 35% of cases. That’s a dramatic escalation: kidney infections during pregnancy can trigger preterm birth, anemia, sepsis, and other serious complications. Treating the silent infection with antibiotics drops the risk of pyelonephritis to just 1% to 4%. This is why professional guidelines strongly recommend a urine culture at one of your first prenatal visits, even if you have no symptoms at all.
Complications of Untreated UTIs
The stakes of an untreated UTI during pregnancy go beyond your own discomfort. Research from Monash University found that women with symptomatic lower urinary tract infections had a preterm birth rate of 12%, compared to 5.1% in women without infections. That translates to roughly 2.5 times the odds of delivering before 37 weeks. The association held even for early preterm birth before 34 weeks, when babies face the most serious health challenges.
Treatment of asymptomatic bacteriuria alone reduces the baseline risk of preterm birth from about 53 per 1,000 women to roughly 14 per 1,000. It also lowers the chance of very low birth weight from approximately 137 per 1,000 to 88 per 1,000. These numbers illustrate why prenatal screening and prompt treatment aren’t optional extras. They are a standard, evidence-backed part of pregnancy care.
How UTIs Are Detected and Treated
Screening typically involves a urine culture collected at one of your early prenatal appointments. A culture is more reliable than a simple dipstick test because it identifies the exact bacterium and its quantity. Counts of 100,000 colony-forming units per milliliter or higher are considered clinically significant. Lower counts may just reflect normal vaginal or vulvar bacteria and don’t need treatment.
If bacteria are found, treatment is a 5 to 7 day course of an antibiotic chosen for safety in pregnancy and effectiveness against the most common culprits. Your provider will select the drug based on your culture results. After treatment, a follow-up culture confirms the infection has cleared. There isn’t strong evidence for or against repeat screening later in pregnancy if your initial culture was negative, so practices vary.
Reducing Your Risk
You can’t eliminate the hormonal and anatomical changes driving UTI risk, but you can minimize bacterial exposure and keep urine flowing efficiently. Staying well hydrated is the simplest and most effective strategy. Aim for six to eight glasses of water daily. Frequent hydration keeps urine dilute and encourages regular bladder emptying, which flushes bacteria before they can establish a foothold.
Other practical steps that reduce risk:
- Empty your bladder frequently and take your time to ensure it’s fully emptied each visit.
- Urinate before and after intercourse to clear bacteria that may have been introduced near the urethra.
- Wipe front to back after using the bathroom, keeping intestinal bacteria away from the urethral opening.
- Wear cotton underwear and avoid tight clothing that traps moisture.
- Skip harsh soaps and douches in the genital area, which can disrupt protective bacteria.
- Limit caffeine and refined sugar, both of which can irritate the bladder or promote bacterial growth.
Cranberry (and blueberry) extracts have some supporting evidence for UTI prevention, though the data in pregnant populations specifically is limited. Vitamin C in modest doses (250 to 500 mg per day) may help by acidifying urine, making it less hospitable to bacteria. These are reasonable additions but not substitutes for hydration, hygiene, and prenatal screening.