Urinary tract infection and pyelonephritis in pregnancy

• Urinary tract infection is one of the most common infections of pregnancy. It ranges from infection of the urine without symptoms, through bladder infection, to kidney infection (pyelonephritis), the most serious form.^[1]
• During pregnancy, urine tends to stagnate, so a silent infection in the urine (asymptomatic bacteriuria) is much more likely than outside pregnancy to reach the kidneys.
• About 2 to 10% of pregnant women have asymptomatic bacteriuria, and if it is not treated, up to 30% develop acute pyelonephritis.^[8]
• Every pregnant woman should be screened with a urine culture early in pregnancy, because treating asymptomatic bacteriuria before it becomes symptomatic prevents kidney infection and may reduce low birth weight and preterm birth.^[5][8]
• Bladder and kidney infections in pregnancy are treated with antibiotics that are safe in pregnancy, chosen from current guidelines, and pyelonephritis usually requires prompt treatment and often admission.^[2][3]

By the end of this page you should be able to:

1. Define asymptomatic bacteriuria, cystitis and pyelonephritis in pregnancy, and explain why pregnancy predisposes to them.
2. Quote the prevalence of asymptomatic bacteriuria and the risk of progression to pyelonephritis if it is untreated.
3. Describe how the screening culture is obtained and what colony count makes it positive.
4. Plan the assessment and first-line antibiotic treatment of cystitis and pyelonephritis in a pregnant woman using current guidelines.
5. Recognise the complications of pyelonephritis in pregnancy.
6. State which antibiotics are avoided or restricted in pregnancy and why.

Asymptomatic bacteriuria is significant quantities of bacteria in the urine without symptoms of urinary infection. The cut-off used in pregnancy is the same as elsewhere: a midstream urine culture showing 10^5 colony-forming units per millilitre (10^5 CFU/mL, often written 100,000 CFU/mL) of a single organism, in a woman with no urinary symptoms.^[2] Some guidelines ask for this on two consecutive specimens when feasible, but one properly collected culture is accepted in practice.^[5]

Acute cystitis is infection confined to the bladder: urgency, frequency and dysuria without fever, flank pain, vomiting or systemic upset. In pregnancy it is usually managed as a complicated lower UTI, which is why a urine culture is required and a longer (7-day) course of antibiotics is used than in non-pregnant women.^[2]

Acute pyelonephritis is infection that has reached the renal parenchyma and pelvis. It presents with fever, rigors, loin or flank pain and tenderness, with or without lower tract symptoms, and is a form of complicated urinary infection.^[3]

UTI is one of the commoner perinatal complications. ACOG estimates that UTIs of all kinds affect roughly 8% of pregnancies.^[1] Asymptomatic bacteriuria occurs in an estimated 2 to 10% of pregnancies.^[8][5]

Pregnancy itself is the major risk factor. From early pregnancy, progesterone relaxes smooth muscle, ureteric peristalsis slows, and the enlarging uterus compresses the ureters, particularly on the right. Urine therefore stagnates in the upper tract, and bacteria that enter the urethra are more likely to travel upwards to the kidney. The risk is worse when the bladder is not emptied fully, when immunosuppression is present, or when the urinary tract is abnormal.

Other risk factors include a previous UTI or bacteriuria, structural or functional abnormalities of the urinary tract (including reflux nephropathy and congenital abnormalities), immunosuppression, diabetes, stone disease and catheter or other instrumentation.^[4]

In one international review, 11 of 13 recommended routine screening of every pregnant woman for asymptomatic bacteriuria, which shows how consistently the evidence is applied.^[5]

Most infections are caused by bowel flora, above all Escherichia coli. Other organisms in pregnancy include Klebsiella pneumoniae, Proteus mirabilis, Staphylococcus saprophyticus, group B streptococcus and enterococci.^[5]

Proteus mirabilis, Morganella and Providencia species tend to produce urease, which can predispose to stones, and nitrofurantoin is not active against them. Group B streptococcal bacteriuria in pregnancy also means the woman should receive intrapartum antibiotic prophylaxis, because of the risk of neonatal infection.^[5]

The usual route is ascent. Bacteria from the perineal skin enter the urethra, colonise the bladder, and, if multiplication outpaces clearance, climb the ureters to the renal pelvis and parenchyma. In pregnancy the usual defences — frequent complete bladder emptying, acid urine and brisk ureteric peristalsis — are weakened, so the same small inoculum is far more likely to ascend.

The asymptomatic stage matters because there is no pain to bring the woman to care: bacteria multiply to significant numbers while she feels well. Cystitis appears when the bladder mucosa is inflamed. Pyelonephritis follows once infection reaches the kidney, producing fever, loin pain and systemic illness; from there, if treatment is delayed, the infection can enter the bloodstream as urosepsis.^[3][6]

One Cochrane review found that antibiotic treatment of asymptomatic bacteriuria reduced the incidence of pyelonephritis (risk ratio 0.23, 95% confidence interval 0.13 to 0.41), and noted a possible reduction in low birth weight and preterm birth, while stressing that the evidence was of low or very low quality.^[8]

Women with asymptomatic bacteriuria have no symptoms at all; they are found because a screening culture was sent.

Cystitis produces irritative voiding symptoms: frequency, urgency, dysuria, suprapubic discomfort, and sometimes haematuria or offensive urine. These may be mild and confused with the frequency of early pregnancy, so it is worth screening the urine when any lower tract symptom is described.

Pyelonephritis is systemic and unmistakable once established: fever and rigors, flank or loin pain (often right-sided), costovertebral angle tenderness, nausea and vomiting, and malaise. Lower urinary symptoms may or may not be present, because infection may have ascended quickly. Abdominal pain can dominate and mimic other causes of acute abdomen in pregnancy.

Ask about:

• previous UTIs or bacteriuria,
• congenital or acquired abnormalities of the urinary tract,
• stones,
• diabetes or immunosuppression,
• features of sepsis (breathlessness, confusion, abnormal temperature, low blood pressure, fast heart rate),
• gestational age, because drug choice changes near term.

• Vital signs, including temperature and oxygen saturation. A pregnant woman with fever and any urinary symptom should be assumed to have pyelonephritis until proven otherwise.^[3]
• Abdominal and flank examination. Tenderness over the kidney, particularly on the right, supports pyelonephritis. Rebound or guarding suggests peritoneal irritation and should prompt consideration of appendicitis or cholecystitis as well.^[3]
• Hydration and perfusion. Dry mucous membranes, tachycardia and hypotension in severe infection indicate sepsis and need immediate treatment.^[6]
• Suprapubic tenderness supports cystitis.
• Obstetric examination as appropriate for gestation, including fetal wellbeing when the woman is unwell.

Several conditions in pregnancy mimic or accompany urinary infection, and missing them is dangerous.

The diagnosis is microbiological. Send a midstream urine (MSU) specimen for culture and susceptibility testing before antibiotics whenever possible.^[2][3] In a febrile or unwell pregnant woman, or when the picture suggests pyelonephritis, take blood cultures and assess renal function before starting treatment, so long as this does not delay intravenous antibiotics.^[3]

• 10^5 CFU/mL (100,000 CFU/mL) is the threshold for asymptomatic bacteriuria in pregnancy.^[2]
• A single clean-catch specimen is usually enough in practice, although a two-specimen standard exists.^[5]
• Pyuria is supportive but not proof of infection; sterile pyuria has other causes.
• Proteinuria or haematuria can accompany infection. Persistent proteinuria after treatment needs reassessment for renal disease or pre-eclampsia rather than being blamed on UTI.^[4]

Treat all documented urinary infection in pregnancy, including asymptomatic bacteriuria, because untreated infection ascends and harms mother and baby.^[1][8] At the same time, use the narrowest appropriate antibiotic and the shortest effective course, because resistance is already common and antibiotic stewardship is part of safe obstetric care.^[2][5]

1. Send a screening culture early in pregnancy.^[5]
2. If positive, give an immediate antibiotic prescription guided by the culture and sensitivity result, rather than waiting for symptoms.^[2]
3. Choose from nitrofurantoin, amoxicillin or cefalexin, selecting according to the sensitivity report and the stage of pregnancy (nitrofurantoin is avoided at term).^[2]
4. Recheck a culture 7 to 14 days after treatment to confirm clearance, and consider monthly cultures thereafter in women at increased risk.^[5]
5. Offer prophylaxis after a single treated episode in women with reflux nephropathy, congenital abnormalities of the kidneys or urinary tract, immunosuppression, or a history of recurrent UTI.^[4]

NICE recommends offering an immediate antibiotic prescription to pregnant women with lower UTI, and obtaining a midstream urine sample before antibiotics are taken for culture and susceptibility testing.^[2] Review and, where needed, change the antibiotic according to the culture result, using a narrow-spectrum drug when possible.^[2]

NICE's antibiotic choices for pregnant women (aged 12 years and over) with lower UTI are:

• First choice: nitrofurantoin 100 mg modified-release twice a day (or, if unavailable, 50 mg four times a day) for 7 days, provided the estimated glomerular filtration rate is 45 mL/min or more. Avoid at term because it may produce neonatal haemolysis.^[2]
• Second choices: amoxicillin 500 mg three times a day for 7 days (only if culture results show susceptibility) or cefalexin 500 mg twice a day for 7 days.^[2]
• Alternative second choices: consult a local microbiologist and choose on the basis of culture results.^[2]

For asymptomatic bacteriuria, NICE allows a choice from nitrofurantoin, amoxicillin or cefalexin on the basis of recent culture and susceptibility results.^[2]

Oral analgesia (paracetamol; ibuprofen if preferred and suitable), adequate fluids and rest are appropriate supportive measures. Cranberry products and urine-alkalinising agents have no evidence for treatment.

NICE recommends obtaining a midstream urine sample before antibiotics and sending for culture and susceptibility testing; reviewing the antibiotic according to the result; and referring to hospital any pregnant woman with suspected pyelonephritis, particularly if she is dehydrated, cannot take oral fluids or medicines, or has features of sepsis.^[3]

First-line treatment choices depend on whether oral or intravenous therapy is needed:

• If she can take oral medicines and is not severely unwell: cefalexin (first-choice oral antibiotic) 500 mg twice or three times a day, up to 1 to 1.5 g three or four times a day in severe infection, for 7 to 10 days.^[3]
• If she is vomiting, unable to take oral antibiotics, or severely unwell: intravenous cefuroxime 750 mg to 1.5 g three or four times a day, with a switch to oral therapy once symptoms settle and she is afebrile.^[3]
• Second choices or combined therapy if susceptibility or sepsis is a concern: consult a local microbiologist.^[3]
• Intravenous antibiotics should be reviewed early, with step-down to oral where possible.^[3]

One international comparison of guidelines found broad agreement that pyelonephritis starts with empiric parenteral therapy and switches to oral after clinical improvement, although the exact first-line drug differs between countries.^[5]

• Nitrofurantoin is widely used and is first line for uncomplicated lower infection when the renal function is adequate, but it is avoided at term because of a risk of neonatal haemolysis, and it is not recommended when the estimated glomerular filtration rate is below 45 mL/min.^[2]
• Penicillins (amoxicillin) are safe, but rising resistance means they are used only when culture shows susceptibility.^[2][5]
• Cephalosporins (cefalexin, cefuroxime) are safe in pregnancy and are the backbone of pyelonephritis treatment in many guidelines.^[2][3][5]
• Trimethoprim is a folate antagonist; many guidelines avoid it in the first trimester, and some restrict it during pregnancy altogether.^[5]
• Co-trimoxazole (trimethoprim/sulfamethoxazole) is avoided by many guidelines in the first and third trimesters.^[5]
• Fluoroquinolones (ciprofloxacin, levofloxacin) are generally avoided in pregnancy. A minority of guidelines (for example, French) use them as a third-line option for resistant organisms, but this is the exception.^[5]
• Gentamicin and other aminoglycosides may be combined for septic pyelonephritis, with appropriate dosing and monitoring of kidney function.^[3][5]
• WHO's AWaRe Antibiotic Book (2022) frames antibiotic selection for urinary infection around appropriate (Access) therapy, local resistance and antimicrobial stewardship. In low-resource settings, if a culture is unavailable, treat empirically with one of the safe first-line agents above and reassess promptly.^[7]

• Progression to pyelonephritis in up to 50% of untreated asymptomatic bacteriuria.^[5]
• Preterm delivery and low birth weight, which ACOG lists among the adverse pregnancy outcomes associated with UTI.^[1]
• Urosepsis and septic shock, the most feared complication of pyelonephritis, especially if treatment is delayed.^[3][6]
• Renal impairment or abscess in severe or complicated infection.^[3]
• Recurrent infection and stone formation, particularly with urease-producing organisms.
• Neonatal infection, including group B streptococcal sepsis, when maternal bacteriuria is caused by group B streptococcus, and the need for intrapartum prophylaxis.

With prompt treatment, most pregnancy UTIs resolve without lasting harm. Screening and treatment of asymptomatic bacteriuria is effective at preventing pyelonephritis, although one Cochrane review described the size of the effect as uncertain because of the low quality of the old trials.^[8] Women with structural abnormalities, immunosuppression or recurrent infection have a higher recurrence rate and need longer follow-up.

• Screen every pregnant woman with a urine culture early in pregnancy.^[5]
• Repeat screening in the third trimester (and monthly in women at high risk) where local policy or risk factors suggest.^[5]
• Prophylaxis after a single treated UTI in women with structural, immunological or recurrent risk factors.^[4]
• Non-drug measures are supported only weakly. NICE found no evidence for cranberry products or urine-alkalinising agents. Sensible advice is to finish all antibiotics, maintain adequate hydration, empty the bladder regularly and seek review promptly if symptoms recur.
• Post-treatment culture 7 to 14 days after therapy, and then monthly until delivery in high-risk women, to detect persistent or recurrent bacteriuria.^[5]

• Children and adolescents. Pregnant young women under 16 are treated on the same antibiotic table as other pregnant women, but children (and most pregnant adolescents) need urine sampling and follow-up in line with the NICE guideline on UTI in children. Infants under 3 months with any UTI need specialist paediatric assessment and intravenous antibiotics.
• Older adults and non-pregnant women. Asymptomatic bacteriuria is not routinely screened for or treated outside pregnancy; the NICE rule is that it is treated in pregnant women only because it is a risk factor for pyelonephritis and premature delivery.^[2]
• Immunosuppressed and renal patients. Women with chronic kidney disease, reflux nephropathy or a renal transplant may need prophylaxis after a single episode and close monitoring of renal function.^[4]
• Pregnancy itself is the special group this whole topic addresses.

When culture and susceptibility testing are not available:

• use a urine dipstick (leucocyte esterase and nitrite) and clinical assessment to select women for treatment,
• treat empirically with a safe first-line agent such as nitrofurantoin (unless at term or renal impairment), cefalexin or amoxicillin, according to local resistance patterns,
• refer any woman with fever, vomiting, or suspected pyelonephritis promptly,
• send a culture later if possible, and treat to the result.

The WHO AWaRe Antibiotic Book (2022) encourages simple, standardised first-line choices and escalation for severe infection, which fits this approach.^[7]

Related MedGrasp topics, listed as plain text (topics may be linked once published and verified): preeclampsia, HELLP and eclampsia; gestational and chronic hypertension in pregnancy; acute kidney injury; chronic kidney disease in pregnancy; sepsis; hyperemesis gravidarum; pregnancy diagnosis and physiology; gestational diabetes.

This page also covers, as book items on the same page, antepartum haemorrhage and ectopic pregnancy as separate topics, and should be read alongside those where relevant.