What is the recommended antibiotic for a pregnant woman with a urinary tract infection (UTI)?

Antibiotic Treatment for UTI in Pregnancy

Nitrofurantoin 100 mg twice daily for 5-7 days is the first-line antibiotic for treating urinary tract infections in pregnant women, with beta-lactams like amoxicillin-clavulanate or cephalexin as safe alternatives when nitrofurantoin is contraindicated. 1

First-Line Antibiotic Selection

Nitrofurantoin is the preferred agent due to its established safety profile, effectiveness in preventing adverse pregnancy outcomes, and lack of resistance development over decades of use. 1, 2 The American College of Obstetricians and Gynecologists specifically recommends this as the first choice for both symptomatic UTIs and asymptomatic bacteriuria in pregnancy. 1

Key Dosing Recommendations:

• Nitrofurantoin: 100 mg orally twice daily for 5-7 days 1
• Amoxicillin-clavulanate or Cephalexin: 4-7 day course 1
• Fosfomycin: Single 3-gram dose (convenient but limited pregnancy data) 1

Critical Contraindications for Nitrofurantoin:

• Avoid near term (38-42 weeks gestation) due to theoretical risk of hemolytic anemia in newborns 1
• Avoid if creatinine clearance <60 mL/min 1
• In these situations, switch to beta-lactams (amoxicillin-clavulanate or cephalexin) 1

Treatment Duration Matters

A 4-7 day course is superior to single-dose therapy for both symptomatic UTIs and asymptomatic bacteriuria in pregnancy. 1 Single-dose regimens show inferior outcomes, including:

• Lower bacteriuria clearance rates (RR 1.28) 1
• Increased risk of low birth weight when compared to 7-day nitrofurantoin courses (RR 1.65) 1

The evidence consistently demonstrates that single-dose therapy is inadequate despite its appeal for compliance. 3, 1

Asymptomatic Bacteriuria Requires Treatment

Unlike non-pregnant populations, pregnant women with asymptomatic bacteriuria must be treated to prevent progression to pyelonephritis and adverse pregnancy outcomes. 3, 1 Untreated asymptomatic bacteriuria leads to pyelonephritis in 20-37% of pregnant women, compared to only 1-4% when treated. 3

Screening Protocol:

• Obtain urine culture at least once in early pregnancy (12-16 weeks) 3
• Pyuria screening is inadequate (only 50% sensitivity) 3
• Use the same antibiotic regimens as for symptomatic UTI 1

Clinical Algorithm

1. Confirm diagnosis with urine culture before or immediately after initiating empiric therapy 1

2. Start empiric treatment immediately with:

   • First choice: Nitrofurantoin 100 mg twice daily for 5-7 days 1
   • If gestational age 38-42 weeks OR CrCl <60: Use amoxicillin-clavulanate or cephalexin for 4-7 days 1
   • If compliance concern: Consider fosfomycin 3g single dose (though less studied) 1

3. Repeat urine culture 7 days after completing therapy to document cure 4

4. For recurrent UTIs during pregnancy: Consider prophylaxis with cephalexin 250 mg or nitrofurantoin 50 mg post-coitally, which reduces UTI recurrence from multiple infections to near-zero 5

Important Caveats

Trimethoprim-sulfamethoxazole should be avoided in the first trimester due to theoretical teratogenic risks with folate antagonism, despite its high efficacy (>80% cure rates). 4 While some older literature mentions its use, current guidelines favor nitrofurantoin and beta-lactams throughout pregnancy. 1

The stakes are high: Untreated UTIs in pregnancy lead to pyelonephritis (which occurs in 1.8-2.1% of all pregnant women without screening programs), premature delivery, low birth weight infants, preeclampsia, and maternal sepsis. 3, 6 Treatment programs that screen and treat asymptomatic bacteriuria reduce pyelonephritis rates to 0.5-0.6%. 3

Resistance patterns matter less with nitrofurantoin compared to other antibiotics, as it maintains effectiveness without inducing R-factor resistance even after 35+ years of clinical use. 2 This makes it particularly valuable in the era of increasing antimicrobial resistance.