Urinary Tract Infections in Pregnancy

Updated: Oct 18, 2017
  • Author: Emilie Katherine Johnson, MD, MPH; Chief Editor: Edward David Kim, MD, FACS  more...
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Practice Essentials

Pregnancy causes numerous changes in the woman’s body that increase the likelihood of urinary tract infections (UTIs). Hormonal and mechanical changes can promote urinary stasis and vesicoureteral reflux. These changes, along with an already short urethra (approximately 3-4 cm in females) and difficulty with hygiene due to a distended pregnant belly, help make UTIs among the most common bacterial infections during pregnancy.

UTIs during pregnancy are associated with risks to both the fetus and the mother, including pyelonephritis, preterm birth, low birth weight, and increased perinatal mortality. In general, pregnant patients are considered immunocompromised UTI hosts because of the physiologic changes associated with pregnancy (see Pathophysiology). These changes increase the risk of serious infectious complications from symptomatic and asymptomatic urinary infections even in healthy pregnant women. (See Urinary Tract Infection in Females.)

Oral antibiotics are the treatment of choice for asymptomatic bacteriuria and cystitis. The standard course of treatment for pyelonephritis is hospital admission and intravenous antibiotics. Antibiotic prophylaxis is indicated in some cases (see Treatment). Patients treated for symptomatic UTI during pregnancy should be continued on daily prophylactic antibiotics for the duration of their pregnancy.

Annual health costs for UTI exceed $1 billion. Although the condition-specific cost of asymptomatic bacteriuria or UTI in pregnancy is unknown, screening for these conditions in pregnant women is cost-effective, compared with treating UTI and pyelonephritis without screening. Goals for future research include targeting low-income groups and women in developing countries for screening and early treatment, as well as determining whether a causal relation exists between maternal UTI and childhood neurologic consequences.

For patient education information, see the Kidneys and Urinary System Center and Pregnancy and Reproduction Center, as well as Urinary Tract Infections, Pregnancy, Bladder Control Problems, and Blood in the Urine.

Definitions of key terms

Urinary tract infection

UTI is defined as the presence of at least 100,000 organisms per milliliter of urine in an asymptomatic patient, or as more than 100 organisms/mL of urine with accompanying pyuria (>7 white blood cells [WBCs]/mL) in a symptomatic patient. A diagnosis of UTI should be supported by a positive culture for a uropathogen, particularly in patients with vague symptoms.

Asymptomatic bacteriuria

Asymptomatic bacteriuria is commonly defined as the presence of more than 100,000 organisms/mL in 2 consecutive urine samples in the absence of declared symptoms. Untreated asymptomatic bacteriuria is a risk factor for acute cystitis (40%) and pyelonephritis (25-30%) in pregnancy. These cases account for 70% of all cases of symptomatic UTI in unscreened pregnant women.

Acute cystitis

Acute cystitis involves only the lower urinary tract; it is characterized by inflammation of the bladder as a result of bacterial or nonbacterial causes (eg, radiation or viral infection). Acute cystitis develops in approximately 1% of pregnant patients, of whom 60% have a negative result on initial screening. Signs and symptoms include hematuria, dysuria, suprapubic discomfort, frequency, urgency, and nocturia. These symptoms are often difficult to distinguish from those due to pregnancy itself.

Acute cystitis is complicated by upper urinary tract disease (ie, pyelonephritis) in 15-50% of cases.

Acute pyelonephritis

Pyelonephritis is the most common urinary tract complication in pregnant women, occurring in approximately 2% of all pregnancies. Acute pyelonephritis is characterized by fever, flank pain, and tenderness in addition to significant bacteriuria. Other symptoms may include nausea, vomiting, frequency, urgency, and dysuria. Furthermore, women with additional risk factors (eg, immunosuppression, diabetes, sickle cell anemia, neurogenic bladder, recurrent or persistent UTIs before pregnancy) are at an increased risk for a complicated UTI.



Infections result from ascending colonization of the urinary tract, primarily by existing vaginal, perineal, and fecal flora. Various maternal physiologic and anatomic factors predispose to ascending infection. Such factors include urinary retention caused by the weight of the enlarging uterus and urinary stasis due to progesterone-induced ureteral smooth muscle relaxation. Blood-volume expansion is accompanied by increases in the glomerular filtration rate and urinary output.

Loss of ureteral tone combined with increased urinary tract volume results in urinary stasis, which can lead to dilatation of the ureters, renal pelvis, and calyces. Urinary stasis and the presence of vesicoureteral reflux predispose some women to upper urinary tract infections (UTIs) and acute pyelonephritis.

Calyceal and ureteral dilatation are more common on the right side; in 86% of cases, the dilatation is localized to the right. The degree of calyceal dilatation is also more pronounced on the right than the left (average 15 mm vs 5 mm). This dilatation appears to begin by about 10 weeks’ gestation and worsens throughout pregnancy. This is underscored by the distribution of cases of pyelonephritis during pregnancy: 2% during the first trimester, 52% during the second trimester, and 46% in the third trimester.

Although the influence of progesterone causes relative dilatation of the ureters, ureteral tone progressively increases above the pelvic brim during pregnancy. However, whether bladder pressure increases or decreases during pregnancy is controversial.

Glycosuria and an increase in levels of urinary amino acids (aminoaciduria) during pregnancy are additional factors that lead to UTI. In many cases, glucose excretion increases during pregnancy over nonpregnant values of 100 mg/day. Glycosuria is due to impaired resorption by the collecting tubule and loop of Henle of the 5% of the filtered glucose, which escapes proximal convoluted tubular resorption.

The fractional excretion of alanine, glycine, histidine, serine, and threonine is increased throughout pregnancy. levels of cystine, leucine, lysine, phenylalanine, taurine, and tyrosine are elevated in the first half of pregnancy but return to reference range levels by the second half. The mechanism of selective aminoaciduria is unknown, although its presence has been postulated to affect the adherence of Escherichia coli to the urothelium.




E coli is the most common cause of urinary tract infection (UTI), accounting for approximately 80-90% of cases. It originates from fecal flora colonizing the periurethral area, causing an ascending infection. Other pathogens include the following [1] :

  • Klebsiella pneumoniae (5%)
  • Proteus mirabilis (5%)
  • Enterobacter species (3%)
  • Staphylococcus saprophyticus (2%)
  • Group B beta-hemolytic Streptococcus (GBS; 1%)
  • Proteus species (2%)

Gram-positive organisms, particularly Enterococcus faecalis and GBS, are clinically important pathogens. Infection with S saprophyticus, an aggressive community-acquired organism, can cause upper urinary tract disease, and this infection is more likely to be persistent or recurrent.

Urea-splitting bacteria, including Proteus, Klebsiella, Pseudomonas, and coagulase-negative Staphylococcus, alkalinize the urine and may be associated with struvite stones. Chlamydial infections are associated with sterile pyuria and account for more than 30% of atypical pathogens.

GBS colonization has important implications during pregnancy. Intrapartum transmission that leads to neonatal GBS infection can cause pneumonia, meningitis, sepsis, and death. Current guidelines recommend universal vaginal and rectal screening in all pregnant women at 35-37 weeks’ gestation rather than treatment based on risk factors.

Incidental documentation of GBS bacteriuria suggests a higher colonization count than is revealed by a screening vaginal or rectal culture. Beta-streptococcal colonization in the urine warrants immediate treatment and antibiotic prophylaxis when the patient presents in labor.

Whether beta streptococci are associated with preterm labor is controversial. In a prospective study, McKenzie et al found no relation between beta-streptococcal bacteriuria and preterm labor, but they described the use of urinary antibodies to identify at-risk women. [2] In 2043 consecutive women, those with E coli antibodies at the initial visit and at 28 weeks’ gestation and women with beta-streptococcal antibodies at 28 weeks’ gestation had a significantly higher chance of preterm delivery.

Cesarean delivery

Cesarean delivery is associated with UTI (increasing the likelihood 2.7-fold), but this association may be confounded by bladder catheterization or prolonged rupture of membranes (PROM). The incidence of symptomatic UTI is 9.3%, and that of asymptomatic bacteriuria is 7.6%.

Orthotopic continent urinary diversion

Many women who, in the past, would have been counseled against pregnancy are now attempting pregnancy. In orthotopic continent diversion (OCD), an ileal-ascending colon conduit is made (OCD, Kock pouch) and reattached to the in situ urethra (OCD) or a continent abdominal stoma (Kock pouch).

Typical candidates are patients born with congenital exstrophy of the bladder in whom primary reconstruction has failed. Recurrent UTI and hydronephrosis are common because of outflow obstruction of the orthotopic stoma secondary to uterine compression or uterine prolapse. Indwelling catheterization of the urethra or continent stoma may be necessary, particularly during the later stages of pregnancy. In rare cases, a percutaneous nephrostomy tube or antegrade passage of a ureteral stent may be indicated.



United States statistics

The frequency of urinary tract infection (UTI) in pregnant women (0.3-1.3%) is similar to that in nonpregnant women. [3] Changes in coital patterns (eg, position, frequency, postcoital antibiotics) can offset recurrence in at-risk individuals.

Overall, UTIs are 14 times more frequent in women than in men. This difference is attributed to the following factors:

  • The urethra is shorter in women
  • In women, the lower third of the urethra is continually contaminated with pathogens from the vagina and the rectum
  • Women tend not to empty their bladders as completely as men do
  • The female urogenital system is exposed to bacteria during intercourse

A difference between pregnant and nonpregnant women is that the prevalence of asymptomatic bacteriuria in pregnant women is 2.5-11%, as opposed to 3-8% in nonpregnant women. In as many as 40% of these cases, bacteriuria may progress to symptomatic upper UTI or pyelonephritis; this rate is significantly higher than that seen in nonpregnant women. [4]

Several patient-level factors are associated with an increased frequency of bacteriuria during pregnancy. Compared with nonindigent patients, indigent patients have a 5-fold increased incidence of bacteriuria. The risk is doubled in women with sickle cell trait. Other risk factors for bacteriuria include diabetes mellitus, [5] neurogenic bladder retention, history of vesicoureteral reflux (treated or untreated), [6] previous renal transplantation, [7] and a history of previous UTIs.

International statistics

Versi et al described a higher prevalence of bacteriuria in pregnant white women (6.3%) than in pregnant Bangladeshi women (2%). [8] Pregnancies that resulted in preterm deliveries were strongly associated with bacteriuria in white women; this association was not observed in Bangladeshi women. The authors hypothesized that the difference could be due to variation in hygiene practices and clothing.

A large population-based study of nearly 200,000 pregnant Israeli women demonstrated a 2.5% rate of asymptomatic bacteriuria [9] and a 2.3% rate of symptomatic UTI. [10] In this population, asymptomatic bacteriuria was found to have an association with multiple pregnancy complications, including hypertension, diabetes, intrauterine growth retardation, prolonged hospitalization, and preterm labor.

The authors suggested that these findings may be a marker for intensity of prenatal care rather than a specific causal effect of the urinary infection. [9] Additionally, their follow-up study examining women with symptomatic UTI showed a clear association between UTI and low birth weight and preterm delivery, a finding consistent with those of multiple previous investigations. [10]

Age- and race-related demographics

The prevalence of UTI during pregnancy increases with maternal age.

A retrospective analysis of 24,000 births found the prevalence of UTI during pregnancy to be 28.7% in whites and Asians, 30.1% in blacks, and 41.1% in Hispanics. When socioeconomic status is controlled for, no significant interracial differences seem to exist. A survey-based analysis of self-reported UTI found similar trends. This study also considered Native American women and found the highest prevalence of UTI in this population (24.2%) as compared with Asian (10.3%), white (16.6%), Hispanic (18.3%), and black (20.3%) women. [11]

UTI is associated with preterm delivery in persons of all races. The adjusted odds ratio in infants with very low birth weight is 2.8 in blacks and 5.6 in whites, adjusted for parity, body mass index, maternal age, marital status, cigarette smoking, education, and prenatal care. The overall relative risk of bacteriuria in blacks or whites is estimated at 1.5-5, and the relative risk of preterm birth in women with bacteriuria is 1.8-2.3.



In most cases of bacteriuria and urinary tract infection (UTI) in pregnancy, the prognosis is excellent. The majority of long-term sequelae are due to complications associated with septic shock, respiratory failure, and hypotensive hypoxia (ie, extremity gangrene).

Maternal UTI has few direct fetal sequelae because fetal bloodstream infection is rare; however, uterine hypoperfusion due to maternal dehydration, maternal anemia, and direct bacterial endotoxin damage to the placental vasculature may cause fetal cerebral hypoperfusion.

Untreated upper UTIs are associated with low birth weight, prematurity, premature labor, hypertension, preeclampsia, maternal anemia, and amnionitis. [12] A retrospective population-based study by Mazor-Dray et al showed that UTI during pregnancy is independently associated with intrauterine growth restriction, preeclampsia, preterm delivery, and cesarean delivery. [9] A prospective cohort study of pregnant patients also suggested an association between maternal UTI and childhood asthma. [13]


 A case-control study demonstrated an increased odds (1.22-fold) of preeclampsia in women with any UTI during pregnancy versus those without UTI. [14] A multicenter retrospective study found that the presence of UTI in pregnancy, particularly in the third trimester, is strongly associated with preeclampsia. Rates of preeclampsia in patients with UTI compared with those without reported UTI were 31.1% vs 7.8%, respectively (P <0.001). The authors hypothesize that the increased maternal inflammatory burden from UTI enhances the risk of preeclampsia. [15]