Which antibiotic is safe to prescribe for a 5‑month pregnant woman with an upper respiratory infection?

Antibiotic Recommendations for URI in 5-Month Pregnant Woman

For a pregnant woman at 5 months (second trimester) with an upper respiratory infection, amoxicillin is the first-line antibiotic choice if bacterial infection is confirmed, with azithromycin as an alternative if penicillin allergy exists.

Key Clinical Decision Points

When to Use Antibiotics

Most URIs are viral and do not require antibiotics 1. Only prescribe antibiotics when:

• Endoscopic evidence of purulence is present 2
• Clinical features suggest bacterial acute rhinosinusitis
• Symptoms persist beyond 10 days or worsen after initial improvement

First-Line Antibiotic Choices

Amoxicillin (Category A/B):

• Safest option across all trimesters 3
• Beta-lactams are considered the gold standard for pregnancy 4, 5
• All penicillins show low risk with no teratogenic effects 3
• Compatible with breastfeeding

Cephalosporins (Category B1):

• Second-line if penicillin intolerance (not true allergy) 6, 7
• First-generation cephalosporins preferred 7
• Compatible in T2/T3 with extensive safety data 3

Azithromycin (Macrolide):

• Use if true penicillin allergy 4
• Erythromycin rated "probably safe" in T2/T3 3
• Azithromycin preferred over erythromycin for better tolerability 4
• Note: Very low risk of hypertrophic pyloric stenosis if used during breastfeeding in first 13 days 3

Antibiotics to AVOID

Absolutely contraindicated:

• Tetracyclines/Doxycycline (Category D): Tooth discoloration, bone growth suppression, maternal fatty liver risk 3, 6
• Fluoroquinolones: Contraindicated throughout pregnancy 2, 6, 7
• Aminoglycosides: Risk of eighth cranial nerve damage and nephrotoxicity 3
• Trimethoprim-sulfamethoxazole: Avoid especially in first trimester 3, 2

Clinical Algorithm

1. Confirm bacterial infection (purulence, prolonged symptoms >10 days)
2. Check penicillin allergy status:
   • No allergy → Amoxicillin (standard dosing)
   • Intolerance only → Cephalosporin (1st generation)
   • True IgE-mediated allergy → Azithromycin
3. Avoid long-term macrolide therapy for chronic rhinosinusitis maintenance 2

Important Caveats

• Most URIs are viral and resolve without antibiotics 1
• Physiologic changes in pregnancy (increased GFR, volume of distribution) may require dose adjustments for renally cleared antibiotics 5
• The risk of untreated bacterial infection (preterm labor, low birth weight) often outweighs antibiotic risks 5, 6
• Do not withhold necessary antibiotics due to pregnancy—untreated infections pose greater fetal risk 6

The 2020 ERS/TSANZ guidelines 3 and 2016 rhinosinusitis pregnancy guidelines 2 provide the strongest evidence base, consistently recommending penicillins and cephalosporins as safest with extensive human safety data across all trimesters.