Jaundice — the yellowing of a newborn's skin and eyes — affects around 60% of term babies and up to 80% of premature babies in the first week of life. Most of the time it is a normal, self-resolving part of the newborn transition. This guide explains what's actually happening, what the bilirubin numbers mean, when treatment is needed, and what genuinely warrants concern.
🌟 Open WiseMama — freeJaundice is caused by a build-up of bilirubin — a yellow pigment produced when red blood cells break down. Understanding why this happens so predictably in newborns makes the whole picture considerably less alarming.
During pregnancy, the baby needs a high concentration of red blood cells to extract oxygen from the maternal blood supply across the placenta. After birth, when the baby is breathing air directly, that extra oxygen-carrying capacity is no longer needed — and the body rapidly breaks down the surplus red blood cells. Bilirubin is the breakdown product of the haemoglobin in those cells.
The liver is responsible for processing (conjugating) bilirubin into a water-soluble form that can be excreted via bile and stools. In newborns, the liver is immature — it is functioning but not yet at full capacity — and the volume of bilirubin arriving from the rapid red cell breakdown temporarily exceeds what it can process. The unconjugated (unprocessed) bilirubin accumulates in the bloodstream and deposits in the skin and the whites of the eyes, causing the characteristic yellow colour.
In a term baby with physiological jaundice, yellowing typically appears on day 2 or 3, peaks between days 3 and 5, and gradually fades over the first 1–2 weeks as the liver matures and catches up. It usually starts on the face, progresses to the chest and abdomen, and in more significant cases extends to the legs and the whites of the eyes.
The majority of newborns who develop jaundice require only monitoring. The minority whose bilirubin rises above treatment thresholds receive phototherapy, which is safe and effective. Serious complications from jaundice are rare in the UK due to systematic monitoring under NICE guidance introduced in 2010.
Bilirubin: The yellow pigment produced when red blood cells break down.
Unconjugated (indirect) bilirubin: The fat-soluble form, before liver processing — the type that builds up in physiological jaundice.
Conjugated (direct) bilirubin: The water-soluble form after liver processing, excreted via bile. A raised conjugated fraction indicates a different, more serious problem.
SBR (serum bilirubin): A blood test measuring bilirubin level — the gold standard for treatment decisions.
TcB (transcutaneous bilirubin): A non-invasive skin probe reading used for initial screening.
The single most important distinction in newborn jaundice is between physiological jaundice — normal, expected, and self-resolving — and pathological jaundice, which has an underlying cause requiring investigation and more active management.
Physiological jaundice is the type affecting the majority of jaundiced newborns. Its defining features are: it appears on day 2 or later (not in the first 24 hours); bilirubin rises gradually, staying within normal ranges for the baby's age; it peaks and then falls without intervention; and the baby is otherwise well — feeding adequately, producing wet and dirty nappies, alert when awake.
Jaundice appearing in the first 24 hours of life is the most important warning sign. This is never physiological and always needs investigation urgently. Causes include: blood group incompatibility between mother and baby (ABO or rhesus incompatibility), which causes accelerated red cell destruction; infection (sepsis); glucose-6-phosphate dehydrogenase (G6PD) deficiency, a genetic condition that increases red cell fragility; and — rarely — structural abnormalities.
Very rapidly rising bilirubin — rising more than 8.5 µmol/L per hour — regardless of when jaundice appeared, also suggests a pathological process rather than physiological accumulation.
Signs of illness alongside jaundice — fever, poor feeding, lethargy, pale or grey colour, or a high-pitched cry — require urgent medical assessment regardless of bilirubin level.
One specific combination requires urgent attention: jaundice accompanied by pale, chalky white, or grey stools (rather than the normal yellow or mustard colour) and dark urine. This pattern suggests that bile is not reaching the gut from the liver — the most concerning cause is biliary atresia, a condition where the bile ducts are blocked or absent. Biliary atresia is rare (around 1 in 8,000–15,000 births) but requires surgical treatment within the first 2–3 months of life. The NHS newborn bloodspot screening programme now includes a jaundice awareness check specifically for this reason. If your baby's stools are persistently pale and they appear jaundiced, contact your GP or midwife the same day.
Visual assessment of jaundice — looking at the skin colour — is not reliable enough for clinical decisions, particularly in babies with darker skin tones where yellowing is harder to see. UK practice uses objective measurement to guide management.
A transcutaneous bilirubinometer is a small probe placed briefly against the skin — typically the forehead or sternum — that measures the amount of yellow light reflected from the skin. It is non-invasive and gives an immediate reading. TcB is reliable for initial screening and monitoring in most babies, but if the reading is in or above the treatment range, a blood test is needed to confirm the level before treatment decisions are made.
A blood sample — usually from a heel prick — measures the actual concentration of bilirubin in the blood. This is the gold standard for treatment decisions. Results are reported in µmol/L (micromoles per litre) and interpreted against an age-specific chart rather than a single fixed number.
This is the detail that most parents find confusing. A bilirubin level of, say, 200 µmol/L means something very different at 24 hours of age versus 72 hours of age. NICE guidelines use charts that plot treatment thresholds against the baby's age in hours — the threshold rises over the first few days because bilirubin is expected to be higher as the normal peak approaches. A level that requires treatment at 30 hours may well be below the treatment line at 60 hours.
When your midwife or neonatologist says "the level is X — it's above/below the line," they mean it is above or below the age-specific treatment threshold on the NICE chart. Asking "is it above or below the treatment line for their age?" gives you more useful information than the number alone.
Treatment thresholds are set lower for babies born before 38 weeks and for babies with additional risk factors — including blood group incompatibility, G6PD deficiency, significant bruising (which releases more bilirubin), and previous sibling with jaundice requiring phototherapy. An immature or stressed liver has less reserve, and the developing brain is more vulnerable to bilirubin at lower concentrations than in a healthy term baby.
Phototherapy is the standard treatment for jaundice above the treatment threshold. It is safe, effective, and well-understood — and it can feel alarming when your newborn is placed under lights with eye shields within days of birth. Knowing what is happening and why makes it considerably easier to be present for.
Unconjugated bilirubin is fat-soluble — it cannot be excreted directly in urine or bile. The liver normally converts it to a water-soluble form, but when the liver is overwhelmed, bilirubin accumulates. Phototherapy bypasses the liver: specific wavelengths of blue light (around 460–490nm) penetrate the skin and convert bilirubin molecules into a structural isomer — a slightly different shape — that is water-soluble and can be excreted directly in bile and urine without needing further liver processing. The light doesn't "break down" bilirubin so much as change its shape so the body can get rid of it.
The baby lies in a cot or incubator under a bank of blue-spectrum lights, wearing only a nappy. Small eye shields — soft padded pads secured with a band — protect the eyes from the direct light. The skin should be as exposed as possible to maximise the light's effect. In some cases a fibreoptic blanket (a "bili blanket") is placed under the baby's back to provide light from below as well — this is "double phototherapy," used when levels are high or rising quickly.
Feeding continues normally during phototherapy. For breastfed babies, frequent feeding is actively encouraged — it stimulates gut motility and helps excrete the transformed bilirubin through the stool. The baby is brought out for feeds and nappy changes, and brief periods of holding are usually permitted. You do not need to leave your baby alone under the lights — being present, talking to them, skin-to-skin during feeds, and engaging with them during breaks all matter and are encouraged.
Bilirubin levels are rechecked regularly — typically every 6–12 hours — to monitor the response. Most babies respond well within 24 hours. Phototherapy is stopped when levels fall below the treatment threshold and remain stable off treatment.
In some NHS areas and via private providers, phototherapy can be delivered at home using a bili blanket, under midwife or neonatal nurse supervision. This is appropriate for babies with moderately elevated bilirubin who are otherwise well and feeding adequately. It allows the family to be at home rather than in hospital while treatment continues, with regular bilirubin checks. Ask your neonatal team if this is available in your area.
In very rare cases — when bilirubin is extremely high and not responding to intensive phototherapy — an exchange transfusion is performed. This involves replacing the baby's blood gradually with donor blood to rapidly reduce bilirubin levels. It is reserved for cases where bilirubin is approaching levels at which neurological risk becomes significant, and it is performed in a neonatal intensive care setting. It is rare in the UK under current monitoring standards.
The relationship between jaundice and feeding is one of the most misunderstood areas — and one where parents frequently receive incomplete or contradictory advice. There are two distinct and separate phenomena: breastfeeding jaundice and breast milk jaundice. They are different in cause, timing, and management.
In the first days after birth, before breastfeeding is established and milk supply comes in, some babies don't receive enough milk and become mildly dehydrated. Dehydration concentrates bilirubin in the blood and reduces gut motility, slowing bilirubin excretion — both of which raise bilirubin levels. This is breastfeeding jaundice: not caused by something in breast milk, but by insufficient breast milk intake.
Management focuses on improving feeding effectiveness — addressing latch and positioning, feeding frequency, and whether any supplementation (top-up feeds) is needed. It is not a reason to switch to formula. Improving the breastfeed is the right approach; if a top-up is recommended, expressed breast milk is the first choice.
Breast milk jaundice is a separate and benign condition that occurs from the end of the first week onwards. A factor present in some women's breast milk — thought to affect bilirubin processing — causes mildly elevated bilirubin that can persist for several weeks. The baby is otherwise completely well: feeding well, gaining weight, producing normal nappies, alert.
Breast milk jaundice does not require treatment and is not a reason to stop breastfeeding. It resolves on its own, typically by 6–8 weeks. The diagnosis is usually clinical — it is considered once infection, biliary pathology, and other causes have been excluded during the prolonged jaundice workup. Some units confirm the diagnosis by a brief "breastfeeding holiday" (switching to formula for 48–72 hours to see if bilirubin falls rapidly), but this is not necessary in all cases and is not recommended unless other causes need to be excluded.
Feed frequently and effectively. Frequent feeding stimulates gut motility, which helps excrete bilirubin in stools. The classic advice to "put the baby in sunlight" has limited evidence and is not a reliable treatment — window glass filters the UV light most, and the baby would need extended, consistent exposure that is impractical. Frequent effective feeding is a far more reliable intervention.
We were told his jaundice might be from my milk and that we should consider stopping breastfeeding to "test" it. Nobody explained that breast milk jaundice is benign or that there was a difference between breastfeeding jaundice and breast milk jaundice. I wish someone had explained the distinction clearly from the start instead of making me feel like my milk was the problem.
Jaundice is considered prolonged when it persists beyond 2 weeks in a term baby, or beyond 3 weeks in a preterm baby. Prolonged jaundice is common — the majority of cases are benign (breast milk jaundice being the most common cause in breastfed babies) — but it requires investigation to rule out the minority of cases with an underlying cause that needs treatment.
NICE guidelines recommend a structured assessment for prolonged jaundice. This typically includes:
These tests are usually arranged by your GP or health visitor at the 2-week check — if your baby is still visibly yellow at 2 weeks and you haven't been contacted about a review, contact your GP proactively.
When the conjugated (direct) bilirubin fraction is raised, it indicates that bile is not moving normally from the liver to the gut. This is categorically different from the unconjugated accumulation of normal physiological jaundice. Causes include biliary atresia, neonatal hepatitis, and rare metabolic conditions. All require specialist investigation and some require urgent intervention — which is why the split bilirubin test is not just a formality but a genuinely important screen.
The vast majority of newborn jaundice is benign. The following situations require same-day or urgent medical contact:
• Yellowing appears in the first 24 hours of life
• Your baby's stools are persistently pale, chalky white, or grey
• Jaundice is still clearly visible at 2 weeks (term) or 3 weeks (preterm) and you haven't had a review
• Your baby seems unwell alongside the jaundice — poor feeding, excessive sleepiness, fewer wet and dirty nappies than expected
• The yellow colour appears to be spreading to the legs or deepening noticeably after day 5
• Your baby is extremely difficult to rouse or unusually limp
• Your baby has a high-pitched or unusual cry
• Your baby is having jerky movements or seizures
• Your baby has a fever (over 38°C in a baby under 3 months requires emergency assessment regardless of jaundice)
These signs alongside jaundice may indicate bilirubin is affecting the brain (acute bilirubin encephalopathy) or that there is an underlying infection. Both are emergencies.
Kernicterus — the chronic form of bilirubin-induced brain damage — causes cerebral palsy, hearing loss, and abnormal tooth enamel development. It results from very high unconjugated bilirubin crossing the blood-brain barrier. In the UK, with systematic NICE-guided monitoring and early phototherapy, kernicterus is rare. The purpose of the monitoring your baby receives in the first days is precisely to prevent this — and it does so effectively.