Sepsis in babies happens when bacteria, and less commonly viruses or fungi, enter a newborn’s bloodstream and trigger a dangerous whole-body immune response. The two most common culprits are Group B Streptococcus (GBS) and E. coli, which together account for more than half of all cases in the first week of life. Globally, neonatal sepsis affects roughly 22 out of every 1,000 live births, with mortality rates between 11% and 19%.
Early-Onset Sepsis: The First 72 Hours
When sepsis develops within the first three days after birth, it’s classified as early-onset. These infections are almost always passed from mother to baby during labor and delivery. Bacteria that normally live in the mother’s birth canal or intestinal tract can reach the baby through the amniotic fluid or during passage through the birth canal.
GBS is the classic cause of early-onset sepsis in full-term infants. In premature babies, E. coli takes over as the leading pathogen, responsible for about 58% of early-onset cases in one decade-long study of preterm infants. Gram-negative bacteria like E. coli dominated roughly 80% of early-onset infections in that group. Less common causes include Listeria, a foodborne bacterium that can cross the placenta during pregnancy.
Late-Onset Sepsis: After the First Week
Sepsis that develops after 72 hours of life tends to come from different sources. Rather than being transmitted during birth, these infections are typically picked up from the baby’s environment, whether that’s a hospital setting or the community. The bacteria involved shift accordingly.
Coagulase-negative staphylococci, a group of bacteria commonly found on human skin, are the most frequent cause of late-onset sepsis. These organisms are especially problematic for premature babies in neonatal intensive care units (NICUs), where they can enter the bloodstream through medical equipment. In the same decade-long study, late-onset infections outnumbered early-onset cases nearly five to one, with 146 confirmed bloodstream infections compared to 31 early-onset cases.
Why NICUs Pose Special Risks
Premature and critically ill babies often need invasive medical devices to survive: central line catheters for nutrition and medication, breathing tubes for respiratory support, and urinary catheters. Each of these creates a direct pathway for bacteria to bypass the skin barrier and enter the bloodstream. A large study of over 6,400 NICU admissions found that respiratory support tripled the risk of a hospital-acquired bloodstream infection. Central line catheters more than doubled the risk, and urinary catheters carried a similar increase.
Prior antibiotic use also raised the odds, likely because antibiotics can wipe out protective bacteria and allow more harmful organisms to take hold. On the protective side, breast milk feeding was associated with fewer infections, likely due to the immune proteins and beneficial bacteria it provides.
Maternal Risk Factors
Several conditions during pregnancy and labor significantly raise a baby’s chances of developing sepsis. The single strongest predictor is premature rupture of membranes, commonly called “water breaking too early.” When the protective amniotic sac opens long before delivery, bacteria from the birth canal can travel upward into the fluid surrounding the baby. Rupture lasting more than 18 hours is considered a major risk factor, and research shows it increases the odds of neonatal sepsis by roughly five times.
Other maternal risk factors include fever during labor (a sign of possible infection in the uterus), foul-smelling amniotic fluid, and cloudy or meconium-stained fluid. Multiple vaginal examinations during labor, particularly three or more, can also introduce bacteria. Carrying twins or multiples adds additional risk.
On the baby’s side, prematurity and low birth weight (under about 4.4 pounds) are consistently linked to higher sepsis rates. Babies who need resuscitation at birth face elevated risk as well, partly because the procedures involved can introduce bacteria and partly because these babies tend to be sicker and more vulnerable from the start.
Why Newborns Are So Vulnerable
A newborn’s immune system is fundamentally different from an adult’s, and not just because it’s “weaker.” Babies produce lower levels of the inflammatory signaling molecules that adults use to mount a rapid defense against invading bacteria. Their white blood cells are also less effective at reaching infection sites. Specifically, neonatal immune cells have about 50% less ability to migrate toward bacteria compared to adult cells. They’re stiffer and less able to squeeze through tiny blood vessels, which means they can actually get stuck and block blood flow, potentially contributing to organ damage.
This combination of sluggish immune signaling and impaired cell movement means that infections can spread through a newborn’s body much faster than they would in an older child or adult. What might be a localized skin infection in a toddler can become a life-threatening bloodstream infection in a newborn within hours.
Signs to Watch For
Sepsis in newborns can be difficult to spot because the symptoms are often vague and overlap with many other conditions. Temperature instability is one of the earliest clues: a baby may develop a fever, but just as often, a dangerously low body temperature is the first sign. Changes in heart rate, either unusually fast or unusually slow, are another red flag. Babies with sepsis often become lethargic, feed poorly, or develop rapid or labored breathing.
Doctors diagnose neonatal sepsis primarily through blood cultures, which identify the specific organism in the bloodstream. They also look at markers of inflammation in the blood, including C-reactive protein levels (values above 6 mg/L raise concern) and the ratio of immature to total white blood cells (a ratio above 0.2 suggests infection). Because blood cultures take time to grow, treatment with antibiotics typically begins before results come back if sepsis is suspected.
How GBS Screening Helps Prevent It
One of the most effective interventions against early-onset sepsis is routine screening for GBS during pregnancy. The CDC recommends that all pregnant women be tested for GBS bacteria during the 36th or 37th week of pregnancy, even if a cesarean delivery is planned. The test is a simple swab.
Women who test positive receive antibiotics during labor, which dramatically reduces the chance of passing the bacteria to the baby. This screening protocol is considered one of the best tools available for preventing GBS disease in the first week of life. It’s recommended for every pregnancy because GBS colonization can change between pregnancies.
Long-Term Effects on Development
Surviving sepsis doesn’t always mean a clean recovery. Research consistently shows that babies who had sepsis face higher rates of developmental delays compared to babies who didn’t. One study found that 24% of babies who survived sepsis caused by GBS had abnormal neurodevelopmental outcomes at one year, with 3.5 times greater odds of neurologic impairment compared to healthy controls.
Early-onset sepsis appears particularly impactful. One study found neurodevelopmental impairment in 24% of affected infants at age two, compared to just 8% of infants without sepsis. When sepsis progresses to meningitis (infection of the membranes surrounding the brain), the risks climb further, with moderate to severe impairment reaching 15%. Premature babies who survive sepsis face the steepest challenges, with some studies reporting substantially higher rates of both mortality and developmental difficulties in the years that follow.