UTI in pregnancy

UTIs affect roughly 10% to 18% of pregnancies and are among the most common maternal infections. They progress in a predictable sequence — asymptomatic bacteriuria, then cystitis, then pyelonephritis — which is why screening and treating the silent stage matters.

Untreated bacteriuria risks maternal sepsis, preterm labour and premature delivery. Around 25% of untreated asymptomatic bacteriuria progresses to symptomatic UTI in pregnancy.

Risk is higher with previous recurrent UTI or a known renal tract abnormality. Other identified risk factors are obesity, low socioeconomic status, young maternal age, nulliparity, diabetes and smoking.

Escherichia coli is the commonest organism, followed by Klebsiella pneumoniae, Proteus, Staphylococcus, Streptococcus and Enterococcus species.

History and examination

Women with bacteriuria are asymptomatic by definition. Ask about previous UTIs or pyelonephritis, and any UTI earlier in this pregnancy.

Cystitis looks the same as outside pregnancy — frequency in about 82%, suprapubic pain or tenderness in about 73%, dysuria in about 55%, plus offensive urine and haematuria. Symptoms alone are unreliable: only about 26% of symptomatic women have a positive culture.

Pyelonephritis is more systemic — flank pain, fever of at least 38 °C, rigors, nausea and vomiting, malaise. It is right-sided more often than left, and bilateral in up to 25%. Check vital signs and costovertebral angle tenderness. Tachycardia or hypotension suggests evolving sepsis.

Uterine irritability commonly accompanies UTI in pregnancy, so assess for contractions and cervical change. Consider appendicitis, cholecystitis, pancreatitis, preterm labour and placental abruption in the differential.

Investigations

• Urine M/C/S for all women in the first trimester (ideally 12–16 weeks) to screen for asymptomatic bacteriuria — repeat if screened earlier.
• Urine M/C/S if symptomatic, or at every antenatal visit if there has been a UTI this pregnancy.
• Culture before starting antibiotics, in pregnancy particularly, given rising resistance.
• If pyelonephritis is suspected, add FBC, electrolytes and creatinine; blood cultures and lactate if sepsis is suspected, taken before antibiotics where feasible.

Routine dipstick at every antenatal visit is no longer recommended — it has a high false-positive rate and poor predictive performance. Nitrite in a symptomatic woman is useful (specificity 98.6%), but a normal dipstick does not exclude infection.

Bacteriuria is defined as a single organism at a colony count of 100,000 CFU/mL or greater on a clean-catch specimen. Contamination is common, and perineal cleansing before collection does not reduce it.

Group B streptococcus in the urine at 100,000 CFU/mL or more is treated as a UTI. At any colony count, GBS in the urine in pregnancy means the woman needs intrapartum IV antibiotic prophylaxis.

Management

Asymptomatic bacteriuria is one of the few groups in which bacteriuria should be treated, because of the risk of pyelonephritis causing preterm birth and low birth weight. Treat per susceptibility and confirm clearance with a repeat culture.

Acute cystitis: nitrofurantoin 100 mg 6-hourly for 5 days, or cephalexin 500 mg twice daily for 5 days; modify to susceptibilities. Trimethoprim is an option in the second and third trimesters. Standard treatment duration is 5 to 7 days for either agent.

Avoid amoxicillin and ampicillin as empiric first-line agents because of high Escherichia coli resistance, and avoid fluoroquinolones in pregnancy.

Nitrofurantoin is pregnancy category A but is not recommended beyond 38 weeks gestation because of the risk of haemolytic anaemia in the neonate, and is best avoided in the first month of breastfeeding for the same reason. Neither nitrofurantoin nor a sulfa derivative should be prescribed in G6PD deficiency, because they can precipitate haemolysis.

Pyelonephritis usually needs hospital admission for IV antibiotics and fluids — arrange ED or maternity unit review. Second- or third-generation cephalosporins are the usual first choice, with an aminoglycoside second line. Nitrofurantoin and fosfomycin do not reach adequate kidney tissue levels and must not be used for pyelonephritis. Prophylactic antibiotics are then continued for the rest of the pregnancy.

Recurrent cystitis: consider prophylaxis if there are risk factors for pyelonephritis (immune compromise, diabetes, neurogenic bladder), guided by sensitivities — cephalexin 250 mg nightly for the pregnancy, or nitrofurantoin 50 mg nightly until 38 weeks. Consider renal tract imaging.

Confirm clearance in every case with a repeat MSU two weeks after finishing antibiotics.