Welcome back to another episode of the OB/GYN Resident Survival Guide, a podcast designed to deliver bite-sized, high-yield clinical pearls you can learn on the run. I’m Dr. KC Miller, OB/GYN, and today we’re breaking down Group B Streptococcus (GBS) prophylaxis in labor—a topic that comes up constantly on Labor & Delivery and one every intern needs to be comfortable managing.

Before diving in, two quick reminders:

The free OB/GYN Intern Starter Pack is available for anyone preparing to start residency on July 1.
Enrollment for OB/GYN Residency Boot Camp opens again this spring. It’s a five-module crash course covering labor, triage, postpartum care, OB emergencies, and clinic—built specifically for incoming interns. The waitlist link is here.

Now, let’s get into the details.

What Is Group B Streptococcus?

Group B Streptococcus (GBS) is a normal component of the gastrointestinal and vaginal microbiome in many patients. Approximately 10–30% of pregnant patients are colonized with GBS in the vagina and/or rectum.

Of those colonized patients, about 50% will transmit GBS to their newborn during labor or after membrane rupture. Without prophylaxis, roughly 1–2% of exposed neonates will go on to develop GBS early-onset disease, which includes sepsis, pneumonia, or meningitis within the first seven days of life.

While the absolute risk may seem low, GBS remains the leading cause of neonatal sepsis in the early newborn period, which is why intrapartum prophylaxis protocols exist.

Who Should Receive GBS Prophylaxis?

Patients should receive intrapartum GBS prophylaxis if they fall into one of two categories:

1. Known GBS Colonization

A patient is considered GBS-positive if:

They had a positive rectovaginal culture collected at or after 36 weeks, or
They developed GBS bacteriuria at any point during pregnancy, which suggests heavy colonization and increased neonatal risk.

2. Unknown GBS Status With Risk Factors

If GBS status is unknown, prophylaxis is recommended when any of the following are present:

Preterm labor (<37 weeks 0 days)
Prolonged rupture of membranes (≥18 hours)
Intrapartum maternal fever (≥38°C / 100.4°F)
History of GBS colonization in a prior pregnancy
History of a neonate affected by GBS early-onset disease
Positive intrapartum nucleic acid amplification test (NAAT), if available

If NAAT testing is negative but the patient is preterm or has risk factors, prophylaxis should still be administered.

How Is GBS Prophylaxis Given?

First-Line Antibiotics

ACOG-supported first-line medications include:

Penicillin G (IV)
Ampicillin (IV)

Antibiotics should be initiated at least 4 hours prior to delivery and continued until birth.

Managing Patients With Penicillin Allergy

Choosing the correct antibiotic depends on the severity of the penicillin allergy. Check out Table 2 in the ACOG practice bulletin for a detailed review of low and high risk penicillin allergy symptoms.

Low-Risk Penicillin Allergy

Cefazolin

High-Risk Penicillin Allergy

(History of anaphylaxis, hives, angioedema, flushing, hypotension, or respiratory distress)

Clindamycin if the isolate is susceptible
Vancomycin if clindamycin susceptibility is unknown or resistant

This is why it’s critical to document penicillin allergy status when ordering GBS cultures in clinic—so the lab can perform clindamycin susceptibility testing when appropriate.

Special Considerations

Pre-labor cesarean deliveries do not require GBS prophylaxis if membranes are intact.
A negative rectovaginal GBS culture is valid for five weeks. If a patient remains pregnant beyond that window, repeat testing is required.
In gray-zone cases (e.g., intact membranes but laboring and awaiting cesarean), management may vary by institution and attending preference.

Key Takeaways

The goal of intrapartum GBS prophylaxis is to reduce the risk of GBS early-onset disease in neonates.

Treat patients in labor if they:

Are GBS-positive during pregnancy, or
Have unknown GBS status with risk factors

Penicillin G or ampicillin are preferred, with cefazolin, clindamycin, or vancomycin used when allergies are present—based on reaction severity and susceptibility testing.

All references, guidelines, and resource links are available in the show notes, including access to the OB/GYN Intern Starter Pack and Residency Boot Camp waitlist.

See you next week for another episode.