Group B strep (GBS) can cause three serious infections in newborns: sepsis (a bloodstream infection), pneumonia, and meningitis. About 1 in every 2,000 babies born in developed countries develops invasive GBS disease, making it one of the leading causes of severe bacterial illness in the first few months of life. Most cases are preventable with antibiotics given during labor, but understanding what GBS does to a baby helps you recognize the stakes and the signs.
How GBS Passes From Mother to Baby
GBS bacteria live naturally in the vaginal and rectal area of roughly 1 in 4 pregnant women. The bacteria themselves don’t usually cause problems for the mother. Transmission happens when the baby passes through the birth canal during delivery and is exposed to vaginal secretions containing the bacteria. If membranes have ruptured (your water has broken), bacteria can also travel upward to reach the baby before delivery begins.
Not every baby exposed to GBS gets sick. Without any preventive treatment, only a small percentage of exposed newborns develop an active infection. But because the consequences can be severe and fast-moving, screening and prevention are standard practice.
Early-Onset Disease: The First Week
Early-onset GBS disease strikes within the first seven days of life, and most cases appear within hours of birth. The baby may show difficulty breathing, grunting, poor feeding, lethargy, or irritability. Skin color changes, particularly a bluish or pale tone, and fever are also common warning signs. These symptoms look nearly identical to other causes of newborn sepsis, which means doctors often can’t tell from symptoms alone whether GBS is the culprit and will begin treatment immediately while running tests.
The respiratory symptoms can be especially confusing in the first day or two. Rapid or labored breathing from a GBS lung infection overlaps with other common newborn breathing issues like transient tachypnea or respiratory distress syndrome in premature infants. This is one reason hospitals monitor newborns closely in the hours after birth, particularly when risk factors are present.
Late-Onset Disease: Week One Through Three Months
Late-onset GBS disease develops between day 7 and day 89 of life, with a median onset around 37 days. The symptoms are similar to early-onset disease: fever, poor feeding, fussiness, and lethargy. But late-onset disease is more likely to present as meningitis, which can add signs like seizures or a bulging soft spot on the baby’s head.
The source of late-onset infection is less clear-cut. While early-onset disease comes from exposure during delivery, late-onset cases may result from continued contact with a colonized mother in the weeks after birth. Breast milk has been proposed as one possible route, though this remains debated. Because antibiotics given during labor don’t eliminate the mother’s colonization entirely, they don’t prevent late-onset disease. The mother can still carry and potentially transmit the bacteria after delivery, and in rare cases, hospital or community environments may also be sources.
What GBS Does to a Baby’s Body
The three main infections GBS causes each affect the baby differently, though they can overlap.
- Sepsis is a bloodstream infection that triggers a body-wide inflammatory response. In a newborn whose immune system is still immature, this can cause dangerously low blood pressure, organ stress, and rapid deterioration. Sepsis is the most common presentation of early-onset GBS.
- Pneumonia is an infection of the lungs. It makes breathing difficult and reduces the baby’s ability to get enough oxygen. A baby with GBS pneumonia typically needs supplemental oxygen and may need help breathing.
- Meningitis is an infection of the membranes surrounding the brain and spinal cord. It causes swelling and pressure in the brain, which can produce seizures, a bulging fontanelle (soft spot), and extreme irritability. Meningitis carries the highest risk of lasting damage.
Long-Term Effects of GBS Meningitis
Babies who survive GBS meningitis face a meaningful risk of lasting developmental problems. A large systematic review found that 32% of meningitis survivors had some form of neurodevelopmental impairment by 18 months of age. About 18%, or roughly 1 in 5 survivors, had moderate to severe impairment.
These impairments span several categories: intellectual and motor disabilities, hearing loss, and vision problems. Some children experience delays in language, social development, or behavior, though these are harder to measure in early follow-up. The severity varies widely. Some children recover fully, while others need ongoing support for learning, movement, or sensory function. Sepsis and pneumonia without meningitis carry a better long-term outlook, though any serious neonatal infection can affect development.
Risk Factors That Raise the Odds
Three factors stand out as increasing the chance a baby will develop GBS disease:
- Positive GBS screening late in pregnancy. A mother who tests positive is carrying the bacteria in quantities that could reach the baby during delivery.
- Fever during labor. Maternal fever can signal an existing infection or inflammation that makes transmission more likely.
- Prolonged rupture of membranes. When 18 hours or more pass between water breaking and the baby’s birth, bacteria have more time to ascend and reach the baby.
Premature birth also increases vulnerability. Preterm babies have less developed immune systems and are less equipped to fight off an infection if exposed.
How Screening and Prevention Work
Current guidelines from the American College of Obstetricians and Gynecologists recommend screening every pregnant woman for GBS during the 36th or 37th week of each pregnancy. This applies even when a cesarean birth is planned. The test is a simple swab of the vaginal and rectal area, with results typically back within a day or two.
If you test positive, you’ll receive intravenous antibiotics during labor. This approach has been remarkably effective. The rate of early-onset GBS disease dropped by more than 80% after widespread adoption of this practice, falling from 1.8 cases per 1,000 live births in the early 1990s to 0.26 per 1,000 by 2010. When antibiotics are given at least four hours before delivery, effectiveness against early-onset disease reaches roughly 91% for full-term infants and 86% for preterm infants.
For women with penicillin allergies, alternative antibiotics are available, though the options are more limited. Some alternatives require additional testing of the GBS bacteria to confirm they’ll be effective, since resistance to certain drugs has increased over time.
One important limitation: labor antibiotics only prevent early-onset disease. They do not protect against late-onset GBS, because they don’t fully clear the mother’s colonization. There is currently no established strategy to prevent late-onset cases, which is why recognizing symptoms in the first three months of life remains important.