A 24-year-old primigravida at 16 weeks of gestation is found to have asymptomatic bacteriuria on routine urine culture during antenatal screening. She has no symptoms of urinary tract infection. What is the drug of choice for treatment?

Explanation

## Asymptomatic Bacteriuria in Pregnancy **Key Point:** Asymptomatic bacteriuria (ASB) occurs in 2–10% of pregnant women and must be treated to prevent pyelonephritis and adverse pregnancy outcomes (preterm labour, low birth weight, maternal sepsis). **High-Yield:** Treatment of ASB in pregnancy reduces the risk of pyelonephritis by ~80% and is a standard of care. Nitrofurantoin is the **drug of choice** for ASB in pregnancy (1st and 2nd trimesters), as recommended by ACOG, WHO, and major obstetric guidelines. ### Drug Selection Rationale | Drug | Safety in Pregnancy | Notes | |------|-------------------|-------| | **Nitrofurantoin** | **First-line (1st & 2nd trimester)** | Excellent urinary concentration; minimal systemic absorption; well-established safety record in pregnancy; avoid at term (≥36 weeks) due to risk of neonatal haemolytic anaemia | | Amoxicillin | Acceptable but NOT first-line | High rates of E. coli resistance (>30–50% in many regions) limit its utility; not recommended as empirical first-line by current guidelines | | Trimethoprim-sulfamethoxazole | Avoid (especially 1st trimester & near term) | Folate antagonist → neural tube defects in 1st trimester; kernicterus risk near term | | Fluoroquinolone | Contraindicated | Cartilage toxicity in animal models; insufficient safety data; reserved only for resistant organisms with no alternatives | **Clinical Pearl:** At 16 weeks gestation (2nd trimester), Nitrofurantoin 100 mg modified-release BD or 50–100 mg QDS for 5–7 days is the recommended first-line regimen for ASB per ACOG Practice Bulletin and NICE guidelines. Amoxicillin, while safe, is no longer preferred empirically due to widespread bacterial resistance. **Why Amoxicillin is NOT the drug of choice:** - *E. coli* (the most common uropathogen) shows >30–50% resistance to amoxicillin in most populations. - Current ACOG, NICE, and WHO guidelines do not list amoxicillin as first-line empirical therapy for ASB in pregnancy. - Nitrofurantoin retains superior efficacy against common uropathogens and has a well-established pregnancy safety profile in the 1st and 2nd trimesters. ### Why Other Options Are Suboptimal - **Amoxicillin:** Safe in pregnancy but high resistance rates make it a poor empirical choice; not first-line per current guidelines. - **Trimethoprim-sulfamethoxazole:** Folate antagonist; teratogenic risk in 1st trimester; kernicterus risk near term. Avoided in pregnancy. - **Fluoroquinolone:** Contraindicated in pregnancy due to cartilage toxicity concerns and insufficient safety data. *Reference: ACOG Practice Bulletin No. 91; NICE Guideline NG112; Williams Obstetrics, 25th edition.*