Bilirubin is a yellow pigment created when the body breaks down old red blood cells. Every newborn produces it, but babies generate significantly more than adults because they break down red blood cells at a faster rate and their livers aren’t yet efficient at processing the excess. When bilirubin builds up in a baby’s blood, it tints the skin and eyes yellow, a condition known as jaundice. About 60% of full-term newborns develop some visible jaundice in their first week of life.
How Bilirubin Forms in Newborns
About 85% of all bilirubin comes from hemoglobin, the oxygen-carrying protein inside red blood cells. When red blood cells reach the end of their lifespan, the body dismantles them and converts the hemoglobin into bilirubin. This freshly made bilirubin (called unconjugated or indirect bilirubin) can’t dissolve in blood on its own. It hitches a ride on a protein called albumin, which carries it to the liver.
Once in the liver, an enzyme transforms the bilirubin into a water-soluble form (conjugated or direct bilirubin) that can be dumped into bile, sent to the intestines, and passed out in stool. That’s why newborn stool has its characteristic yellow-green color. Some bilirubin in the intestines gets reabsorbed back into the bloodstream and returned to the liver for another round of processing, a loop known as enterohepatic circulation.
Newborns face a double challenge. They break down red blood cells faster than adults, producing more bilirubin to process. At the same time, the liver enzyme responsible for converting bilirubin is present at low levels in the first days of life. This mismatch between production and processing is why nearly all newborns have higher bilirubin levels than older children or adults. Bruising from delivery, or blood collecting under the scalp during birth, can push bilirubin even higher by adding extra red blood cells to break down.
Unconjugated vs. Conjugated Bilirubin
When your baby’s doctor orders a bilirubin test, the results typically show two types. The distinction matters because each one points to different concerns.
Unconjugated (indirect) bilirubin is the form that hasn’t yet been processed by the liver. This is the type that rises in normal newborn jaundice. It’s fat-soluble, which means at very high levels it can cross from the bloodstream into the brain, where it may cause damage. The vast majority of newborn jaundice involves this type and resolves without complications.
Conjugated (direct) bilirubin is the water-soluble form that the liver has already processed. Elevated conjugated bilirubin is never considered a normal finding in newborns. It doesn’t pose the same direct risk to the brain, but it can signal an underlying problem with the liver or bile ducts that needs investigation. A conjugated level above 2 mg/dL, or more than 20% of the total bilirubin, is considered elevated.
How Bilirubin Is Measured
Hospitals commonly use two methods to check bilirubin. The first is a small handheld device pressed against the baby’s skin, usually on the forehead or chest. This transcutaneous measurement is painless and gives results almost instantly. Studies show it’s highly sensitive for ruling out significant jaundice: if the skin reading is low, a blood test usually isn’t necessary.
The second method is a blood test, either from a heel prick or a small vein draw. This total serum bilirubin measurement is the gold standard for accuracy and is used to confirm any concerning skin readings. If the transcutaneous device shows an elevated number, a blood sample typically follows to get the precise level and determine whether treatment is needed.
Breastfeeding Jaundice vs. Breast Milk Jaundice
Two distinct forms of jaundice are linked to nursing, and they have different causes and timelines.
Breastfeeding jaundice appears in the first three days of life and peaks around days 5 to 15. It happens when a baby isn’t getting enough milk yet, whether from delayed milk production or difficulty latching. The resulting dehydration and low calorie intake slow the intestines down, allowing more bilirubin to be reabsorbed into the bloodstream instead of passed in stool. Babies with breastfeeding jaundice often show mild weight loss and signs of dehydration. The fix is increasing the volume and frequency of feeds, not switching to sugar water, which doesn’t reduce jaundice.
Breast milk jaundice is a separate condition that shows up after the first four to seven days and can persist for weeks. The cause isn’t fully understood, but substances naturally present in some mothers’ breast milk appear to slow down the liver enzyme that processes bilirubin. Other components in breast milk may increase bilirubin reabsorption from the intestines. Babies with breast milk jaundice are otherwise healthy, gaining weight normally, and feeding well. The jaundice typically resolves on its own, though it can linger longer than most parents expect.
What Jaundice Looks Like
The classic sign is a yellow tint to the skin and the whites of the eyes, usually appearing within two to five days after birth. You can check at home by gently pressing on your baby’s forehead or the tip of the nose. If the spot looks yellow when you briefly press and release, mild jaundice is likely present. If the skin simply looks momentarily lighter than usual before returning to its normal color, jaundice probably isn’t significant. Always check in good lighting, ideally near a window with natural daylight, since artificial light can mask the yellow hue.
Jaundice that starts at the face and moves downward toward the chest, belly, arms, and legs generally reflects rising bilirubin levels. Yellowing that extends beyond the face and chest, or deepens noticeably over hours, warrants a call to your baby’s doctor. The same goes for yellowing in the whites of the eyes.
When Bilirubin Gets Dangerously High
In rare cases, unconjugated bilirubin climbs high enough to cross into brain tissue. Levels above 20 to 25 mg/dL raise concern, though there’s no single cutoff that guarantees harm. Several factors affect risk: premature babies are more vulnerable, as are babies experiencing infection, low oxygen, or dehydration, all of which can make it easier for bilirubin to penetrate the brain.
The early warning signs of bilirubin-related brain irritation include extreme sleepiness, poor feeding or weak sucking, and a floppy or limp body. As the condition progresses, a baby may develop a high-pitched cry, become irritable, and arch the back with the head tilted backward. In the most severe stage, seizures, muscle rigidity, and unresponsiveness can occur. This progression, from lethargy to rigidity, can happen over hours to days.
Long-term damage from very high bilirubin, historically called kernicterus, can result in permanent hearing loss, movement disorders, and intellectual disability. This outcome is preventable with timely monitoring and treatment, which is why hospitals routinely screen bilirubin before discharge and schedule early follow-up visits.
How High Bilirubin Is Treated
The most common treatment is phototherapy, where the baby lies under special blue-spectrum lights (sometimes called “bili lights”) wearing only a diaper and protective eye covers. The light energy converts bilirubin in the skin into a form the body can excrete without needing the liver to process it first. Most babies need phototherapy for one to two days, and parents can typically hold and feed the baby during brief breaks from the lights.
For extremely high bilirubin that doesn’t respond to phototherapy, a procedure called exchange transfusion can rapidly remove bilirubin from the bloodstream. This is rare and reserved for the most severe cases. In both scenarios, frequent feeding helps move bilirubin through the intestines faster and reduces the amount reabsorbed back into the blood.
Practical Signs to Watch For
In the first two weeks of life, keep an eye on your baby’s skin color daily. Increasing yellow color, especially spreading from the face down to the trunk and limbs, is the most visible sign that bilirubin may be climbing. Other signals include a baby who is unusually sleepy and hard to wake for feedings, feeds poorly or refuses to latch, produces fewer wet or dirty diapers than expected, or develops a high-pitched or unusual cry. Any combination of deepening jaundice with feeding difficulties or behavioral changes is worth a prompt call to your pediatrician.