Seizures in babies have many possible causes, and the trigger depends heavily on the baby’s age. In newborns (the first 28 days of life), the vast majority of seizures result from acute brain injury, most commonly a lack of oxygen during or around birth. In older infants, fevers become the leading trigger, with febrile seizures most common between 12 and 18 months of age. Less frequently, infections, genetic conditions, and structural brain differences are responsible.
Oxygen Deprivation During Birth
The single most common cause of seizures in newborns is oxygen deprivation, a condition doctors call hypoxic-ischemic encephalopathy. This happens when something disrupts blood flow or oxygen delivery to a baby’s brain during labor, delivery, or the period immediately before or after birth. Possible triggers include a compressed or knotted umbilical cord, placental abruption, prolonged labor, or a drop in the mother’s blood pressure.
When oxygen supply drops, brain cells can’t maintain their normal electrical balance. Calcium floods into neurons, setting off a chain reaction that overexcites brain circuits and damages cells. Certain areas of the brain, particularly regions involved in movement and sensory processing, are especially vulnerable. Most seizures caused by oxygen deprivation appear within the first 24 to 72 hours after birth and are a key early signal that the brain has been injured.
Infections That Reach the Brain
Bacterial meningitis and viral infections of the brain are serious causes of neonatal seizures. Group B Streptococcus (GBS) is the most commonly identified pathogen, responsible for roughly 50% of bacterial meningitis cases in U.S. newborns. E. coli accounts for about 20%, and Listeria makes up another 5 to 10%, often transmitted before birth through the placenta.
On the viral side, herpes simplex virus (HSV) is a particular concern. About 95% of neonatal HSV infections are acquired during delivery. HSV-2 carries the highest risk of neurological damage, including seizures, developmental disability, and abnormal brain growth. Human parechovirus-3 has also been increasingly identified as a cause of meningitis in newborns.
Seizures from meningitis or encephalitis tend to appear in the first week or two of life. Prolonged seizures lasting more than 72 hours in the setting of meningitis are a warning sign of more severe brain injury.
Febrile Seizures in Older Infants
Once babies pass the newborn period, fever-triggered seizures become far more common than any other type. Febrile seizures affect children between 6 months and 5 years old, with the highest risk window between 12 and 18 months. They occur when a child’s body temperature rises, typically above 100.4°F (38.0°C), though even a low-grade fever can be enough to set one off.
What surprises many parents is that the height of the fever matters less than the speed at which it rises. A child spiking a quick fever from a common ear infection or a viral illness can have a febrile seizure before a parent even realizes the child is sick. The vast majority of febrile seizures are “simple,” meaning they last under five minutes, involve the whole body, and don’t recur within 24 hours. They look alarming but do not cause brain damage or increase the risk of epilepsy in most children.
Genetic Causes
About 15% of neonatal seizures stem not from an acute injury but from an underlying epilepsy syndrome. Within that group, roughly 42% have a purely genetic origin, 41% involve a structural brain malformation, and about 9% have both.
One well-studied example is benign familial neonatal seizures, caused by mutations in the KCNQ2 or KCNQ3 genes. These genes provide the blueprint for potassium channels in the brain, which act like valves that help neurons reset after firing. When the channels don’t work properly, neurons become overly excitable and fire too easily. About 70% of families with this condition carry a mutation in one of these two genes. Despite the word “benign” in the name, seizures can be frequent in the newborn period, though most children outgrow them within weeks to months.
Genetic seizure syndromes overall account for up to 6% of all neonatal seizures. They are relatively rare, but identifying them matters because the treatment approach can differ from seizures caused by an acute injury.
Structural Brain Differences
Some babies are born with brain structures that didn’t develop as expected during pregnancy. These include conditions where the brain’s outer layer (the cortex) forms abnormally, areas of the brain fail to develop, or fluid-filled spaces in the brain are enlarged. These structural differences can create zones of abnormal electrical activity that trigger seizures, sometimes starting before birth.
Bleeding in or around the brain is another structural cause, particularly in premature infants whose blood vessels are fragile. Intraventricular hemorrhage, where bleeding occurs in the brain’s fluid-filled chambers, is one of the more common causes of seizures in babies born very early.
Metabolic and Other Triggers
Newborns are vulnerable to metabolic shifts that wouldn’t typically cause problems in older children. Low blood sugar (hypoglycemia), low calcium, low magnesium, and low sodium can all trigger seizures in the first days of life. These are often correctable once identified and tend to resolve quickly with treatment.
Drug withdrawal is another recognized cause. Babies born to mothers who used opioids, benzodiazepines, or certain other substances during pregnancy can develop neonatal abstinence syndrome, which sometimes includes seizures in the first few days after birth.
Infantile Spasms: A Specific Pattern to Watch For
Infantile spasms are a distinct seizure type that typically appears between 2 and 12 months of age, peaking between 4 and 8 months. They look different from what most people picture when they think of a seizure. During a spasm, the body stiffens suddenly for less than one second. The arms may fly up or extend, the head may drop forward, and the back may arch. Sometimes the only visible sign is an eye roll or a subtle crunch of the stomach muscles, paired with a brief look of surprise.
What makes infantile spasms distinctive is that they come in clusters, often right after a baby wakes up. A parent might see a series of quick head bobs or body jerks repeated over several minutes. Because individual spasms are so brief and subtle, they’re frequently mistaken for startle reflexes or colic. Early recognition is critical because infantile spasms are associated with developmental regression, and outcomes improve significantly when treatment begins quickly.
How Seizures in Babies Can Look
Seizures in newborns and young infants often don’t resemble the dramatic convulsions people associate with seizures in adults. Many neonatal seizures are “subtle,” meaning their visible signs are easy to miss. These can include rhythmic twitching of one limb or the face, repetitive bicycling movements of the legs, sustained eye deviation to one side, lip smacking, or episodes of stiffening or posturing.
Some seizures have no visible signs at all and can only be detected with continuous brain monitoring. This is one reason hospitals use EEG monitoring in at-risk newborns: a baby can appear calm while abnormal electrical discharges are occurring in the brain. If you notice any repetitive, rhythmic, or unusual movements in your baby that you can’t interrupt by gently repositioning or touching the limb, those movements are worth reporting to your pediatrician promptly.