What oral antibiotic is safe and appropriate for an 8‑week pregnant woman with a urinary tract infection?

Treatment of UTI at 8 Weeks Gestation

For an 8-week pregnant woman with a urinary tract infection, initiate nitrofurantoin 100 mg twice daily for 7 days after obtaining a urine culture, or alternatively use cephalexin 500 mg four times daily for 7 days if nitrofurantoin is contraindicated. 1

Immediate Diagnostic Steps

• Obtain a urine culture before starting antibiotics to guide therapy and confirm the diagnosis, as dipstick testing has only 50% sensitivity for detecting bacteriuria in pregnancy. 1
• Do not wait for culture results to initiate treatment—start empiric therapy immediately after collecting the specimen, as delayed treatment increases the risk of pyelonephritis and adverse pregnancy outcomes. 1

First-Line Antibiotic Options

Nitrofurantoin (Preferred)

• Nitrofurantoin 100 mg twice daily for 7 days is the first-line recommendation for UTI in the first trimester. 1
• Alternative dosing: nitrofurantoin macrocrystals 50-100 mg four times daily for 7 days. 1
• Nitrofurantoin is safe throughout pregnancy, with a retrospective analysis of 91 pregnancies showing no fetal toxicity or drug-related adverse events. 1
• Critical contraindication: Never use nitrofurantoin for suspected pyelonephritis, as it does not achieve therapeutic blood concentrations. 1

Fosfomycin (Alternative)

• Fosfomycin trometamol 3 g single oral dose is an acceptable alternative to nitrofurantoin for uncomplicated lower UTI. 1
• The European Association of Urology recommends fosfomycin for treating both asymptomatic bacteriuria and symptomatic UTIs in pregnant women. 1

Cephalosporins (Second-Line)

• Cephalexin 500 mg four times daily for 7-14 days is appropriate when nitrofurantoin or fosfomycin cannot be used. 1
• Other options include cefpodoxime or cefuroxime for 7-14 days. 1
• Cephalosporins achieve adequate blood and urinary concentrations and have excellent safety profiles in pregnancy. 1

Antibiotics to Avoid in First Trimester

• Trimethoprim-sulfamethoxazole is contraindicated in the first trimester due to potential teratogenic effects (neural tube defects and cardiac malformations). 1
• Fluoroquinolones (ciprofloxacin, levofloxacin) must be avoided throughout pregnancy due to potential adverse effects on fetal cartilage development. 1
• The European Medicines Agency and multiple guidelines explicitly recommend against fluoroquinolone use in pregnancy. 1

Treatment Duration and Follow-Up

• Standard treatment duration is 7 days for symptomatic UTI, though 7-14 days is acceptable depending on clinical response. 1
• Obtain a follow-up urine culture 1-2 weeks after completing treatment to confirm cure. 1
• If symptoms persist or recur within 2 weeks, obtain repeat culture with susceptibility testing and retreat with a 7-day course of an alternative antibiotic. 1
• Monthly urine screening throughout pregnancy is recommended to detect recurrent bacteriuria. 1

Special Considerations

If Culture Grows Proteus mirabilis

• Proteus mirabilis is intrinsically resistant to nitrofurantoin—switch immediately to a cephalosporin (e.g., cephalexin) guided by susceptibility testing. 1
• Proteus accounts for less than 10% of uncomplicated UTIs but produces urease, warranting evaluation for urinary stones if infection persists. 1

If Group B Streptococcus (GBS) is Isolated

• Women with GBS bacteriuria at any concentration during pregnancy require treatment at the time of diagnosis plus intrapartum GBS prophylaxis during labor. 1
• GBS bacteriuria is a marker for heavy genital tract colonization—these women do not need vaginal-rectal screening at 35-37 weeks. 1

Critical Clinical Context

• Untreated bacteriuria increases pyelonephritis risk 20-30 fold (from 1-4% with treatment to 20-35% without treatment). 1
• Treatment reduces premature delivery and low birth weight infants in pregnant women with UTIs. 1
• Pregnancy is the one clinical scenario where even asymptomatic bacteriuria must always be treated due to significant risk for progression to pyelonephritis and adverse pregnancy outcomes. 1

Common Pitfalls to Avoid

• Do not rely on negative dipstick to rule out UTI—the presence of symptoms in pregnancy warrants culture and empiric treatment regardless of dipstick results. 1
• Do not use nitrofurantoin if upper tract infection (pyelonephritis) is suspected—switch to parenteral ceftriaxone or cefepime. 1
• Do not perform surveillance urine testing or treat asymptomatic bacteriuria repeatedly after the initial screen-and-treat approach, as this fosters antimicrobial resistance. 1
• Do not classify pregnant women with UTIs as "complicated" unless they have structural/functional urinary tract abnormalities or immunosuppression, as this leads to unnecessary broad-spectrum antibiotic use. 1