First-Line Treatment for Group B Streptococcal (GBS) Infection in Pregnancy

Group B Streptococcus (Streptococcus agalactiae) is a leading cause of neonatal sepsis, pneumonia, and meningitis when transmitted from a colonized mother to the neonate during labor and delivery. Intrapartum antibiotic prophylaxis (IAP) is the mainstay of preventing early-onset neonatal GBS disease, and the choice of antibiotic is well standardized by ACOG/CDC guidelines.

Analysis of Options

• Penicillin (correct answer): Penicillin G is the recommended first-line agent for GBS intrapartum prophylaxis because GBS remains uniformly susceptible to it, it has a narrow spectrum (minimizing disruption of other flora and resistance selection), and it achieves adequate levels in the fetal compartment. Ampicillin is an acceptable alternative, but penicillin is preferred due to its narrower spectrum.
• Azithromycin: Not used for routine GBS prophylaxis. Azithromycin is a macrolide reserved mainly for other indications (e.g., chlamydia, certain respiratory infections) and GBS resistance to macrolides is increasingly common, making it an unreliable choice for prophylaxis.
• Doxycycline: Contraindicated in pregnancy because tetracyclines deposit in fetal bone and teeth, causing dental staining and potential inhibition of bone growth; it is never used for GBS prophylaxis in pregnant women.
• Vancomycin: Reserved as an alternative only in women with a high-risk penicillin allergy (anaphylaxis) when the GBS isolate is resistant to clindamycin or susceptibility is unknown. It is not first-line therapy.

Conclusion

Given GBS's reliable susceptibility and safety profile in pregnancy, penicillin is the correct first-line agent for intrapartum GBS prophylaxis, with alternatives (ampicillin, cefazolin, clindamycin, or vancomycin) reserved for specific circumstances such as penicillin allergy.