What Is Bilirubin?
Bilirubin is a yellow substance produced when the body breaks down old red blood cells.
Normally, the liver processes bilirubin so it can be removed from the body.
Newborn babies often have higher bilirubin levels because their bodies are breaking down red blood cells after birth, and their livers are still developing the ability to process bilirubin efficiently.
When bilirubin builds up in the blood, it can cause jaundice, which gives the skin and whites of the eyes a yellow appearance.
You may hear your baby's healthcare team refer to bilirubin as bili.
Why Does the NICU Check Bilirubin Levels?
Bilirubin levels help the healthcare team determine whether your baby has jaundice that requires treatment.
A certain amount of bilirubin is expected in newborns, particularly during the first several days after birth.
However, levels that become too high can be harmful.
The NICU team monitors bilirubin to determine:
Whether the level is rising or falling
Whether treatment is needed
How well phototherapy is working
Whether additional testing is necessary
How Is Bilirubin Measured?
Bilirubin can be measured in two ways.
Blood Test
A small blood sample is collected to measure the bilirubin level. This is called a total serum bilirubin (TSB).
Blood testing provides the measurement used to make important treatment decisions.
Skin Measurement
Some babies have bilirubin checked using a handheld device placed against the skin.
This is called a transcutaneous bilirubin (TcB) measurement.
It estimates bilirubin without requiring a blood sample, but a blood test may still be necessary to confirm the result.
Skin measurements may be less reliable in certain circumstances, including during or shortly after phototherapy.
What Is a Normal Bilirubin Level?
There is no single bilirubin number that is considered normal or safe for every newborn.
The healthcare team interprets your baby's level based on several factors:
Gestational age at birth
Age in hours or days
Whether your baby was born prematurely
How quickly the bilirubin level is rising
Other medical conditions or risk factors
For example, a bilirubin level that does not require treatment in a healthy full-term newborn may require closer monitoring or treatment in a very premature baby.
The number itself is only part of the picture. Your baby's age and medical condition are equally important.
Why Are Premature Babies More Likely to Have High Bilirubin?
Premature babies are especially likely to develop elevated bilirubin levels because their bodies are still maturing.
Several factors contribute:
Immature Liver Function
The liver may not process bilirubin as efficiently as it does in a more mature baby.
Red Blood Cell Breakdown
Newborns naturally break down red blood cells after birth, producing bilirubin.
Feeding Challenges
Babies who are not yet receiving adequate nutrition may eliminate bilirubin more slowly.
Immature Intestinal Function
Some bilirubin can be reabsorbed from the intestines, contributing to higher levels.
Because premature babies may be more vulnerable to bilirubin-related complications, their treatment thresholds are often lower than those used for full-term newborns.
Why Does My Baby's Bilirubin Level Keep Changing?
Bilirubin levels often rise during the first several days after birth before gradually decreasing.
This happens as the baby's liver becomes more efficient at processing bilirubin and the body adjusts to life outside the womb.
Your baby's level may also change because of:
Feeding and stooling patterns
Increased breakdown of red blood cells
Bruising from delivery
Blood group incompatibility
Infection or other medical conditions
The NICU team looks at the pattern of bilirubin measurements, not just one result.
What Happens if My Baby's Bilirubin Is Too High?
The most common treatment for elevated bilirubin is phototherapy.
Phototherapy uses special blue light to change bilirubin into forms the body can eliminate more easily.
During treatment, the healthcare team repeats bilirubin testing to evaluate how well the treatment is working.
In rare cases of severe hyperbilirubinemia, additional treatments may be needed.
Your baby's team will determine the safest approach based on the bilirubin level and your baby's overall condition.
What Is the Difference Between Total and Direct Bilirubin?
You may hear the NICU team discuss different types of bilirubin.
Total Bilirubin
This measures the overall amount of bilirubin in the blood.
It is commonly used to evaluate newborn jaundice and guide phototherapy decisions.
Direct Bilirubin
Direct bilirubin is bilirubin that has been processed by the liver.
An elevated direct bilirubin level can indicate a different problem involving the liver or bile flow.
Direct bilirubin elevations are evaluated differently from the more common newborn jaundice associated with elevated indirect bilirubin.
If your baby's direct bilirubin is elevated, the healthcare team may recommend additional testing.
Why Is Monitoring Bilirubin Important?
Most newborn jaundice is temporary and resolves as the baby's body matures.
However, very high levels of certain forms of bilirubin can affect the brain.
This is why the NICU team carefully monitors bilirubin levels and treats them when necessary.
With appropriate monitoring and treatment, serious bilirubin-related complications are uncommon.
What Can Parents Ask?
If your baby's bilirubin is being monitored, consider asking:
What is my baby's current bilirubin level?
Is the level rising or falling?
What bilirubin level would require treatment for my baby?
How often will bilirubin be checked?
Is my baby at higher risk because of prematurity?
Will the bilirubin level need to be checked after discharge?
What Should Parents Know?
Bilirubin is a normal substance produced when red blood cells break down.
Newborns, especially premature babies, may have difficulty processing and eliminating bilirubin during the first days of life.
A higher bilirubin level does not automatically mean something is seriously wrong.
The important questions are how high the level is for your baby's age, how quickly it is changing, and whether treatment is needed.
Your baby's healthcare team uses bilirubin measurements to guide treatment and keep your baby safe.
