Low blood sugar in newborns, called neonatal hypoglycemia, most commonly happens because of a mismatch between a baby’s energy supply and demand in the hours after birth. During pregnancy, the fetus receives a steady stream of glucose through the placenta. Once the umbilical cord is cut, the baby must regulate blood sugar on its own for the first time, and several conditions can make that transition difficult. In the first four hours of life, a blood glucose level below 25 mg/dL is considered too low. After four hours and through the first day, the threshold rises to 45 mg/dL.
How a Baby’s Blood Sugar Works at Birth
Inside the womb, a baby doesn’t need to produce its own glucose. It flows directly from the mother’s bloodstream across the placenta. In the final weeks of pregnancy (the third trimester), the fetus builds up energy reserves in the form of glycogen stored in the liver and body fat. These reserves act as a fuel tank the baby draws from during the gap between birth and establishing regular feedings.
After birth, the baby’s body must switch on several processes at once: breaking down those glycogen stores, producing new glucose in the liver, and eventually absorbing glucose from breast milk or formula. When any part of this system is immature, overwhelmed, or underdeveloped, blood sugar drops.
Maternal Diabetes Is the Most Common Cause
Babies born to mothers with gestational diabetes or pre-existing diabetes face the highest risk. The mechanism is straightforward: glucose crosses the placenta freely, so when the mother’s blood sugar runs high during pregnancy, the baby is bathed in excess glucose for months. The baby’s pancreas responds by producing extra insulin to handle the load. After birth, the glucose supply from the placenta stops abruptly, but the baby’s pancreas keeps churning out high levels of insulin. That surplus insulin pulls blood sugar down fast.
This is why infants of diabetic mothers are routinely screened for low blood sugar during the first 24 hours of life, even if they appear perfectly healthy. The excess insulin typically tapers off within a day or two as the baby’s system recalibrates, but the early hours can be a vulnerable window.
Size at Birth Matters
Babies who are unusually small or unusually large for their gestational age are both at increased risk, though for different reasons.
Small-for-gestational-age babies (those below the 10th percentile in weight) simply haven’t built up enough glycogen and fat reserves. Their fuel tank is small, so it empties quickly. These babies may also have less mature metabolic pathways for producing glucose on their own.
Large-for-gestational-age babies (above the 90th percentile) are often large because they were exposed to high glucose levels in the womb, frequently from maternal diabetes that may not have been diagnosed. Like infants of diabetic mothers, they tend to overproduce insulin. Their size can actually mask the underlying metabolic problem.
Premature Birth and Low Energy Reserves
Babies born before 37 weeks are at elevated risk because they missed out on the critical third-trimester period when the body stockpiles glycogen and fat. A baby born at 34 weeks, for example, has significantly fewer energy reserves than a full-term baby. On top of that, the liver enzymes responsible for manufacturing new glucose may not be fully functional yet. Premature infants also tend to feed less effectively, which delays the intake of calories that would help stabilize blood sugar.
Birth Stress and Oxygen Deprivation
A difficult delivery can drain a newborn’s glucose reserves rapidly. When a baby experiences oxygen deprivation before, during, or just after birth (called perinatal asphyxia), the body’s metabolic demand spikes. The brain, which depends heavily on glucose for energy, consumes it faster under stress. At the same time, reduced oxygen impairs the baby’s ability to use glucose efficiently and disrupts the hormonal signals that regulate glucose production. Elevated stress hormones like cortisol further throw off glucose metabolism. Babies with low Apgar scores (below 7 at ten minutes) are screened for this reason.
Excess Insulin From Genetic Conditions
In a small number of babies, low blood sugar isn’t a temporary transition problem. It’s caused by a genetic condition called congenital hyperinsulinism, where the pancreas produces too much insulin regardless of how much glucose is in the blood. Mutations in at least nine genes can cause this condition, with one gene (ABCC8) accounting for roughly 40% of known cases.
Congenital hyperinsulinism causes frequent, recurring episodes of low blood sugar that don’t resolve on their own the way transitional hypoglycemia does. It’s relatively rare, but it’s one of the most important causes to identify early because it requires ongoing treatment. Physical clues that might point to a genetic cause include midline facial defects or unusually small genitalia in boys, both of which prompt screening.
Metabolic Disorders That Affect Glucose
Several inherited metabolic conditions can interfere with the body’s ability to store, release, or produce glucose. These are individually uncommon but collectively represent an important category of causes:
- Glycogen storage diseases: The liver stores glucose as glycogen but can’t release it back into the bloodstream properly. Multiple types exist, and hypoglycemia is the primary symptom in most of them.
- Fatty acid oxidation disorders: When glucose runs low, the body normally switches to burning fat for energy. Babies with these disorders can’t make that switch, so blood sugar drops faster during any gap in feeding.
- Disorders of gluconeogenesis: The liver can’t manufacture new glucose from other building blocks. These babies develop low blood sugar whenever they go without food for even moderate periods.
- Galactosemia and hereditary fructose intolerance: These conditions impair the processing of specific sugars, disrupting the liver’s normal glucose-releasing functions.
These metabolic conditions typically cause persistent or recurrent low blood sugar episodes rather than a single dip in the first day of life. Many are detected through newborn screening programs.
Medications the Mother Received
Certain drugs given to the mother before delivery can affect the baby’s blood sugar. Beta-blockers (used for high blood pressure), terbutaline (used to slow preterm labor), and antenatal steroids given within 48 hours of delivery are all recognized risk factors. These medications can cross the placenta and temporarily alter how the baby’s body handles glucose or insulin.
How Low Blood Sugar Looks in a Newborn
Many babies with low blood sugar show no obvious symptoms at all, which is exactly why at-risk infants are screened with a heel-prick blood test rather than watched for signs. When symptoms do appear, they can include jitteriness or tremors, pale or bluish skin, rapid breathing or pauses in breathing, poor feeding, vomiting, floppiness, difficulty staying warm, irritability, and in severe cases, seizures.
These signs overlap with many other newborn conditions, so blood sugar is typically checked alongside other tests whenever a baby seems unwell.
Does It Cause Lasting Harm?
For most babies, transient low blood sugar that’s caught and treated promptly does not lead to long-term problems. A large prospective study following children to school age found that neurocognitive impairment rates were essentially the same in children who had neonatal hypoglycemia and those who didn’t (51% vs. 50% in an at-risk population). However, the picture changes when episodes are severe (below 36 mg/dL) or happen three or more times. Children who experienced severe or recurrent hypoglycemia showed worse visual motion perception and a higher risk of emotional and behavioral difficulties at school age.
The key factor is not whether blood sugar dipped once, but how low it went and how many times it happened. This is why hospitals screen frequently in the first 24 hours for at-risk babies, catching drops early before they become severe or repeated.