What is the preferred oral antibiotic for a 28‑week pregnant patient with an uncomplicated bacterial infection and no known drug allergies?

Preferred Antibiotic for a 28-Week Pregnant Patient

For a pregnant patient at 28 weeks with an uncomplicated bacterial infection and no drug allergies, amoxicillin or ampicillin are the preferred first-line oral antibiotics, as penicillins have decades of documented safety in pregnancy and excellent activity against common obstetric pathogens. 1, 2

First-Line Antibiotic Selection

Penicillins as Gold Standard

• Penicillins (amoxicillin, ampicillin) represent the safest and most extensively studied antibiotics in pregnancy, with decades of clinical experience documenting their pharmacokinetics and overall fetal safety 2
• Penicillins provide excellent coverage against Group A and Group B streptococci, which are critical pathogens in obstetric infections 1
• Ampicillin is particularly valuable for enterococcal infections, especially urinary tract infections common in pregnancy 1

Cephalosporins as Equally Safe Alternatives

• First-generation cephalosporins (cephalexin) are also considered first-line agents during pregnancy with equivalent safety profiles to penicillins 2, 3
• More commonly used cephalosporins should be given priority over newer agents due to more extensive safety data 3
• Cephalosporins are appropriate when broader gram-negative coverage is needed 1

Infection-Specific Considerations

For Urinary Tract Infections

• Ampicillin or amoxicillin remain excellent choices for uncomplicated UTIs in pregnancy 1
• Cephalexin provides broader gram-negative coverage if needed 3
• If Group B Streptococcus is isolated from urine at any concentration, immediate treatment is required AND the patient will need intravenous intrapartum prophylaxis during labor regardless of current treatment 4

For Respiratory Infections

• Amoxicillin or amoxicillin-clavulanate are preferred for community-acquired pneumonia or sinusitis 2
• Azithromycin is reserved for atypical pathogens like Mycoplasma pneumoniae 1

For Skin and Soft Tissue Infections

• Cephalexin or dicloxacillin (if methicillin-susceptible Staphylococcus aureus is suspected) are appropriate choices 3

Antibiotics to Avoid in Pregnancy

Absolutely Contraindicated

• Tetracyclines should never be used after the fifth week of pregnancy due to effects on fetal bone and tooth development 3
• Fluoroquinolones are contraindicated as a precautionary measure, though the absolute risk remains debated 3, 5
• Aminoglycosides should be avoided except in life-threatening infections due to nephrotoxicity and ototoxicity risks 3

Use with Caution or as Second-Line

• Sulfonamides, trimethoprim, and cotrimoxazole are second-line agents and should be avoided in the first trimester and near term 3
• Metronidazole is permitted during pregnancy but requires strict verification of indications 3
• Lincosamides (clindamycin) should only be used if penicillins, cephalosporins, and erythromycin have failed 3

Critical Clinical Pitfalls

Common Errors to Avoid

• Do not withhold necessary antibiotic treatment due to pregnancy concerns—serious maternal infections pose greater risks to both mother and fetus than appropriate antibiotic use 3
• Avoid using broader-spectrum agents when narrow-spectrum penicillins are adequate, as this promotes resistance 6
• Never assume that treating a UTI eliminates the need for intrapartum GBS prophylaxis if GBS was isolated—this is a dangerous and common error 4

Special Considerations at 28 Weeks

• At 28 weeks gestation, the patient is in the third trimester when infections can precipitate preterm labor 3
• If the patient has any signs of preterm labor with unknown GBS status, she should receive immediate intravenous GBS prophylaxis 4
• Serologic testing for syphilis should be repeated at 28-32 weeks in high-risk populations 7

Algorithm for Antibiotic Selection

1. Identify the infection type (UTI, respiratory, skin/soft tissue, etc.)
2. Choose amoxicillin or ampicillin as first-line for most common bacterial infections 1, 2
3. Use cephalexin if broader gram-negative coverage is needed or if penicillin allergy is reported but not confirmed 3
4. Reserve azithromycin for atypical pathogens (Chlamydia, Mycoplasma) 1
5. Ensure appropriate duration (typically 7-10 days for most infections) and monitor for clinical improvement at 48-72 hours 8
6. Document any GBS isolation and ensure communication to the delivery team for intrapartum prophylaxis planning 4