Jaundice causes a yellow tint in a newborn’s skin and eyes, driven by a buildup of bilirubin, a pigment the body produces when it breaks down old red blood cells. In most babies, jaundice is harmless and clears on its own within the first two weeks of life. In rare cases where bilirubin climbs too high, it can cross into the brain and cause lasting neurological damage.
Why Newborns Are Prone to Jaundice
Babies are born with a high volume of red blood cells, and fetal red blood cells have a shorter lifespan than adult ones. As those cells break down rapidly after birth, the process releases bilirubin. At the same time, a newborn’s liver is still immature. It has low levels of the proteins and enzymes needed to convert bilirubin into a water-soluble form the body can flush out through stool and urine. The result is a temporary bottleneck: bilirubin is produced faster than the liver can process it, and levels rise.
This is why some degree of jaundice shows up in the majority of full-term newborns during their first week. It’s a normal transitional phase, not a disease. Bilirubin typically peaks around day three to five of life, then gradually falls as the liver matures and feedings increase.
How Jaundice Looks on a Baby
The yellowing usually appears on the face first. As bilirubin levels climb, the color spreads downward to the chest, belly, arms, and legs. The whites of the eyes and the area under the tongue often turn yellow too. If your baby has darker skin, the yellowing may be harder to spot on the body, but checking the eyes and under the tongue gives a clearer picture.
The progression itself is a rough visual guide to severity. Yellowing limited to the face generally corresponds to lower bilirubin levels. When the color reaches the arms and legs, levels are higher and worth checking.
Physiological vs. Pathological Jaundice
Most newborn jaundice is physiological, meaning it’s the expected result of normal red blood cell turnover and an immature liver. It shows up after the first 24 hours, peaks in the first week, and resolves without intervention.
Pathological jaundice is different. It appears within the first 24 hours of life, rises faster, and reaches higher levels. Common causes include blood type incompatibility between mother and baby (such as Rh or ABO incompatibility), which accelerates red blood cell destruction, and certain genetic conditions that affect how quickly the liver processes bilirubin. Premature babies are at higher risk because their livers are even less developed.
Breastfeeding and Jaundice
Two distinct types of jaundice are linked to breastfeeding, and they work through different mechanisms.
Breastfeeding jaundice happens in the first week when milk intake hasn’t been fully established. If a baby isn’t getting enough milk, stool output drops. Since bilirubin leaves the body primarily through stool, delayed passage of meconium (the first dark stool, which is packed with bilirubin) allows bilirubin to be reabsorbed back into the bloodstream. The fix is straightforward: more frequent feedings. Breastfeeding can and should continue.
Breast milk jaundice appears later, usually in the second week or beyond, and can persist for several weeks. The exact cause isn’t fully understood, but substances in breast milk may slow the liver’s ability to clear bilirubin. This type is generally mild. Occasionally, a temporary pause in breastfeeding for 12 to 48 hours helps confirm the diagnosis. If a pause is needed, pumping or hand expression keeps milk supply intact.
What Happens When Bilirubin Gets Too High
Unconjugated bilirubin is fat-soluble, which means it can cross the blood-brain barrier. At very high levels, it deposits in brain tissue and damages neurons. This condition is called kernicterus, or bilirubin encephalopathy, and it progresses in stages.
Early on, a baby becomes extremely sleepy, feeds poorly, has floppy muscle tone, and stops reacting to loud sounds. In the middle stage, irritability increases, crying becomes high-pitched, and muscles tighten. In the late stage, the baby stops feeding, the body arches backward with the neck bent, and seizures begin. Kernicterus can lead to hearing loss, cerebral palsy, cognitive impairment, permanent brain damage, or death.
Kernicterus is rare precisely because jaundice is monitored closely in the first days of life. But it is entirely preventable with timely treatment, which is why recognizing rising bilirubin matters.
How Bilirubin Is Measured
Most hospitals check bilirubin levels before a newborn goes home. The quickest method uses a handheld sensor pressed against the baby’s skin. This transcutaneous reading is painless and good at ruling out dangerous levels. Its sensitivity ranges from 74% to 100%, meaning it reliably catches high readings. However, when a skin reading comes back elevated, a blood test (a small heel prick) confirms the exact number. Blood testing remains the gold standard.
There is no single bilirubin number that triggers treatment for every baby. Current guidelines from the American Academy of Pediatrics use hour-specific curves that account for the baby’s exact age in hours, gestational age, and individual risk factors. A bilirubin level that’s safe at 48 hours may not be safe at 24 hours.
How Phototherapy Works
The primary treatment for elevated bilirubin is phototherapy, commonly called “light therapy” or “bili lights.” The baby lies under a special blue-spectrum light, sometimes with a light pad underneath as well, wearing only a diaper and protective eye covers.
The light does something the liver can’t yet do efficiently. It transforms bilirubin molecules in the skin into water-soluble forms that the body can excrete through bile and urine without needing the liver to process them first. Some of these altered molecules are also broken down into colorless compounds that pass out in urine. The process works quickly. Many babies need only one to two days of phototherapy before levels drop enough to stop.
In the most severe cases, where bilirubin rises dangerously fast despite light therapy, an exchange transfusion (replacing a portion of the baby’s blood) may be necessary. This is extremely rare.
Warning Signs to Watch At Home
Most babies are discharged before bilirubin peaks, so knowing what to monitor matters. Yellow coloring that spreads below the chest, difficulty waking the baby for feeds, very few wet or dirty diapers, a high-pitched or unusual cry, and a baby who feels stiff or unusually floppy are all signals that bilirubin may be climbing too high. Jaundice that appears within the first 24 hours of life is always considered urgent. If your baby’s skin looks increasingly yellow, especially in the belly, arms, or legs, a bilirubin check can quickly clarify whether treatment is needed.