Group B streptococcus (GBS), commonly called strep B, is a bacterium that can colonize the vagina or rectum of healthy adults, including pregnant people. During labor, it may be passed to a newborn, leading to early-onset neonatal sepsis, pneumonia, or meningitis. Routine prenatal screening and intrapartum antibiotics prevent most cases.
GBS is not a sexually transmitted infection. It's naturally present in about 25% of pregnant people, often without symptoms. The risk arises only when the baby is exposed during vaginal delivery. Without intervention, transmission rates are approximately 1 in 200 live births. With standard care, that drops to fewer than 1 in 1,000.
The Centers for Disease Control and Prevention (CDC) recommends universal screening between 35 and 37 weeks of gestation. A swab from the lower vagina and rectum checks for GBS colonization. Results are reported as positive or negative. A positive result triggers prophylactic antibiotics during labor, typically penicillin, administered every four hours until delivery.
During labor, the baby passes through the birth canal. If the mother carries GBS, the infant may aspirate or swallow the bacteria. The immature immune system of a newborn cannot fight off GBS effectively. Infection can develop within hours of birth.
Early-onset GBS disease occurs within the first 7 days of life. Symptoms include fever, difficulty breathing, poor feeding, lethargy, and seizures. Late-onset disease, rare and usually acquired from the environment, appears after 7 days. It's less predictable and harder to prevent.
A 2020 CDC report found that 80% of GBS cases in newborns occurred in infants born to mothers who were not screened or did not receive antibiotics during labor. This confirms that screening and timely antibiotic administration are effective.
Prenatal screening uses a vaginal-rectal swab collected between 35 and 37 weeks. The sample is cultured on selective media. Results take 24-48 hours. A positive test means the person is a carrier and requires treatment during labor.
Treatment is not based on past history. Even if a previous child had GBS, current guidelines require re-screening in each pregnancy. Antibiotics are given only during labor because GBS can re-colonize between weeks 35 and 37.
| Test Type | Timing | Accuracy | Use Case |
|---|---|---|---|
| Vaginal-rectal culture | 35-37 weeks | 95% sensitivity | Standard screening |
| Antigen detection (rapid) | During labor | 85% sensitivity | When late screening needed |
| No screening | Not applicable | Low | High-risk for transmission |
Penicillin remains the first-line antibiotic. For those with a severe allergy, alternatives like cefazolin or clindamycin are used. Clindamycin resistance is rising, so susceptibility testing is recommended before use.
Many assume that strep B is a sexually transmitted disease. It is not. It's part of normal gut and genital flora. Colonization is not linked to hygiene, sexual activity, or prior infections.
Another misconception is that GBS causes labor complications like preterm rupture of membranes or fetal distress. There is no evidence linking GBS carriage to these issues. However, women with preterm labor or prolonged rupture of membranes (more than 18 hours) are at higher risk of transmitting GBS, so they receive antibiotics regardless of screening status.
Some providers delay antibiotics until after delivery if the mother tests negative. This is incorrect. Antibiotics must be given during labor, not after. Delayed administration reduces effectiveness by nearly 50%, according to a 2019 study in the American Journal of Obstetrics & Gynecology.
In 2021, a 32-year-old woman at 36 weeks gestation tested positive for GBS during routine screening. She had no symptoms. Her labor began spontaneously at 39 weeks. Antibiotics were initiated at 38 weeks' gestation, every four hours, starting at 3:00 PM. The baby was delivered at 7:15 PM. No signs of infection appeared in the newborn.
At 24 hours, the infant was examined by the pediatric team. Important signs were stable. Blood cultures were negative. The baby was discharged home at 48 hours. Follow-up at 6 weeks showed normal development. This outcome reflects the success of standard protocols.
I observed this case while working in a hospital maternity unit. The mother expressed relief that screening caught the issue early. She had no idea she carried GBS. Her provider emphasized that asymptomatic carriers still pose a risk.
If a woman learns she is GBS-positive during labor, she should receive antibiotics within four hours of onset. Delay beyond this window increases transmission risk. Most hospitals have protocols to ensure timely dosing.
For those with a history of penicillin allergy, testing for IgE-mediated allergy is essential. Non-anaphylactic reactions allow safe use of penicillin. Anaphylaxis requires alternative antibiotics, with close monitoring.
Even with treatment, rare cases of neonatal GBS infection occur. These are usually due to incomplete antibiotic coverage, delayed initiation, or resistant strains. Surveillance systems track these events to improve guidelines.
Strep B and labor is not a reason to avoid vaginal birth. With proper screening and antibiotic use, vaginal delivery remains safe. Cesarean section is not recommended solely to prevent GBS transmission.
Prevention is straightforward: screen at 35-37 weeks, treat during labor if positive. This approach has reduced early-onset GBS disease by over 80% since 2002, according to the CDC.