Group B Streptococcus (GBS) is a bacterium that lives harmlessly in the gut and genital tract of roughly 1 in 4 adults. In pregnancy, it matters because it can be transmitted to the newborn during delivery and, in a small fraction of cases, cause serious infection in the first week of life. The good news: a single screening swab and a course of antibiotics in labor have cut early-onset neonatal GBS disease by more than 80 percent since universal screening began in the late 1990s.
This post explains how screening works, what a positive result actually means, and what to expect during labor under current ACOG guidance.
Why GBS matters in newborns
Early-onset GBS disease occurs in the first 6 days of life and presents most often as sepsis, pneumonia, or meningitis. Before universal screening, the incidence was roughly 1.7 per 1,000 live births in the United States. After implementation of risk-based screening (1996) and then universal culture-based screening at 35 to 37 weeks (2002), incidence fell to around 0.2 to 0.3 per 1,000 live births, according to CDC ABCs surveillance data.
Late-onset disease (7 days to 3 months) is unaffected by intrapartum antibiotics and remains roughly 0.3 per 1,000 live births. The screening program targets the early-onset window because that is when antibiotics during labor make a measurable difference.
How screening works
ACOG Committee Opinion 797 (2020) updated the timing recommendation to 36 weeks, 0 days through 37 weeks, 6 days. The earlier guideline (35 to 37 weeks) was extended slightly because culture results best predict colonization status within 5 weeks of delivery.
The test itself:
- A combined vaginal-rectal swab. Both sites are needed because GBS can colonize either or both, and a vaginal-only swab misses cases.
- Self-collected swabs are acceptable and have similar sensitivity to provider-collected swabs in most studies.
- Culture is the standard. Some labs add a rapid PCR for late-presenting labor without prior screening, though sensitivity is slightly lower.
A positive result is recorded in the prenatal chart and used to plan intrapartum antibiotic prophylaxis (IAP).
When to skip the swab and treat anyway
Some situations bypass screening because risk is high enough to warrant treatment regardless:
- GBS bacteriuria during the current pregnancy. A urinary tract infection with GBS, even at low colony counts, signals heavy colonization. ACOG recommends IAP without further swab.
- A previous infant with invasive GBS disease. The risk to subsequent infants is elevated.
- Unknown GBS status at labor with risk factors: preterm labor under 37 weeks, rupture of membranes for 18 hours or more, intrapartum fever (100.4 F or 38 C and higher), or a positive intrapartum NAAT test.
In these scenarios, antibiotics are started without waiting for culture.
What intrapartum antibiotic prophylaxis looks like
The first-line regimen is IV penicillin G, 5 million units initial dose, then 2.5 to 3 million units every 4 hours until delivery. Ampicillin is an alternative.
Adequate prophylaxis is defined as at least one dose of penicillin, ampicillin, or cefazolin at least 4 hours before delivery. Babies whose mothers received less than 4 hours are considered partially treated and are observed more closely.
For penicillin allergy, ACOG 797 specifies:
- Low-risk allergy (rash without anaphylaxis, angioedema, respiratory distress): cefazolin.
- High-risk allergy (anaphylaxis or severe reaction): clindamycin if isolate is sensitive, otherwise vancomycin. Susceptibility testing should be requested when the swab is sent.
Vancomycin dosing was updated in 2020 to 20 mg/kg every 8 hours (max 2 g per dose), reflecting better penetration to the fetus.
What this means for the laboring person
If you are GBS positive:
- Your labor team starts IV antibiotics when you arrive in active labor or when membranes rupture, whichever comes first.
- The IV does not interfere with most birth plans. You can move, use a tub between doses (with the line capped), labor in any position. Doses run for 15 to 30 minutes.
- Inductions and scheduled cesareans both follow the same logic: cesarean before labor and before membrane rupture has very low GBS transmission risk and does not require prophylaxis. If labor starts or membranes rupture before the planned cesarean, antibiotics are given.
If you decline antibiotics, the baby is monitored more closely after birth and may have additional blood work in the first 24 to 48 hours. This is a personal decision with measurable tradeoffs, and many providers will support a clearly informed choice while explaining the absolute risk numbers.
What this means for the newborn
Most babies born to GBS-positive parents who received adequate IAP need no special intervention beyond standard newborn care. The 2019 AAP guideline simplified neonatal management:
- Well-appearing term infant, mother received adequate IAP: routine newborn care, observe for 36 to 48 hours.
- Well-appearing infant, inadequate IAP, no other risk factors: clinical observation, typically no labs.
- Sick-appearing infant or additional risk factors (chorioamnionitis, prolonged ROM): blood culture and empirical antibiotics, decisions individualized.
Late-onset GBS (after the first week) is not prevented by intrapartum antibiotics. Symptoms include fever, poor feeding, and lethargy in the 1 to 12-week range — call your pediatrician for any of these signs regardless of GBS status.
Common myths
“GBS is an STI.” It is not. GBS is part of the normal flora for many adults and is not sexually transmitted in any clinically meaningful way.
“A positive test means I did something wrong.” It does not. Colonization is common and not related to hygiene or behavior.
“I can clear it with probiotics or vinegar washes.” No high-quality evidence supports any of these interventions for preventing neonatal disease. The intervention that works is IV antibiotics during labor.
“I tested negative last pregnancy, so I do not need to test now.” Status changes between pregnancies. Each pregnancy gets its own swab.
Where this fits in your timeline
The GBS swab is one of several late-pregnancy tests. Use the Pregnancy Week Calculator to track when you are in the 36 to 37-week window, and review what to expect in the third trimester alongside our post on gestational age vs fetal age for clarity on the dating language clinicians use.
If your due date is uncertain, an early ultrasound CRL measurement is the most reliable anchor — see ultrasound due date and CRL.
Questions worth asking your provider
- When am I scheduled for the GBS swab?
- Do I have a documented penicillin allergy on file? If so, what type of reaction?
- If I go into labor before the swab, what is the plan?
- If I am scheduled for a cesarean and labor starts early, what changes?
- What is your unit’s protocol if I receive less than 4 hours of antibiotics before delivery?
Bringing these questions to the 36-week visit lets your team document a clear plan before things move quickly.
The bottom line
GBS screening at 36 to 37 weeks is a low-effort, high-yield test. A positive result is not a crisis — it identifies you for a routine intervention (IV antibiotics in labor) that has cut neonatal early-onset disease dramatically. ACOG Committee Opinion 797 and the 2019 AAP guideline together set out a clear algorithm that your prenatal and delivery teams will follow. Knowing the plan in advance reduces surprises in labor and lets you focus on the parts of birth that are not about a swab result.