When a Newborn's Jaundice Runs in the Family

What four 2026 studies found about inherited causes of newborn jaundice, and what parents can reasonably ask about them

Most newborn jaundice is harmless and passes within a week or two, but a small group of babies turn yellow early, stay yellow longer, or need more treatment than expected — and in many of those babies the reason is something they inherited. Four studies published in 2026 in Thailand, Malaysia, and China found inherited causes in a meaningful share of jaundiced newborns, most of which were never identified before the baby went home.

Jaundice — the yellow tint to a newborn's skin and eyes — comes from bilirubin, a yellow-orange substance the body makes when it breaks down old red blood cells. Every newborn produces bilirubin faster than a brand-new liver can clear it, which is why most babies look at least a little yellow in their first week. Hospitals watch it closely because very high bilirubin can, rarely, injure the brain, and the treatment is straightforward: a special blue light, called phototherapy, that changes bilirubin into a form the baby can get rid of. What gets much less attention is the question underneath the treatment — why did this particular baby's bilirubin climb?

The question that often goes unanswered

Parents of a jaundiced newborn are usually told one of a handful of familiar explanations: the baby's blood type didn't match the mother's, the baby was born early, feeding is still getting established, or there is some bruising from delivery. These are real and common causes. But many babies are treated, monitored, and discharged with no cause identified at all, under a label that describes the jaundice rather than explains it.

For a long time that was an acceptable outcome, and for most babies it still is. Careful research decades ago showed that breastfed babies often stay mildly yellow for weeks with no consequence whatsoever [[1]], which reassured everyone and reduced a lot of unnecessary testing. The main national guideline in the United States, updated in 2022, focuses on measuring bilirubin at the right times and treating at the right level, and it names only one inherited condition — G6PD deficiency — as a specific risk factor to watch for [[2]]. Worldwide, though, severe jaundice is far more common in regions where far less research has been done [[3]], and the 2026 studies come from exactly those places.

The two conditions at the centre of these studies

The first is G6PD deficiency. G6PD is an enzyme — a tiny molecular tool — that protects red blood cells from chemical stress. Babies who inherit a faulty version have red cells that can break apart more easily, releasing a surge of bilirubin. It is the most common enzyme disorder in the world, affecting hundreds of millions of people, and it is far more common in boys than girls because of how the gene is inherited [[4]]. Many countries already test every newborn for it.

The second is less familiar: hereditary elliptocytosis, a condition in which the scaffolding that holds a red blood cell's shape is slightly faulty. The cells become less flexible and wear out faster, which again releases bilirubin. Most people who carry it live entirely normal lives and never know [[5]]. The catch, and it is an important one, is that this condition is very hard to spot in a newborn: the distinctive oval-shaped red cells that give it its name usually don't appear under the microscope until four to six months of age [[6]]. In Thailand, researchers established that almost all cases trace back to three specific spelling changes in a single gene called SPTB [[7]], and that a laboratory test could check for all three in about four hours [[8]]. That test is what made these studies possible.

What the researchers did

Two teams in Thailand approached the same question from opposite directions. At Srinagarind Hospital, part of Khon Kaen University in northeastern Thailand, doctors looked back at 1,584 newborns who had been treated for jaundice over three years and checked which of them turned out to have hereditary elliptocytosis [[9]]. At Ramathibodi Hospital in Bangkok, a second team went forward instead: between March and November 2024 they tested the umbilical cord blood of 1,505 consecutive newborns — nearly every baby born there in that period — and then followed what happened to their jaundice [[10]].

A third team, at Hospital Tuanku Jaafar in Seremban, Malaysia, studied 1,042 newborns admitted for jaundice treatment and compared those with G6PD deficiency to those without [[11]]. A fourth, at Yuebei People's Hospital in northern Guangdong, China, took 56 infants whose jaundice had no explanation after all the usual tests and read 161 different genes known to affect jaundice, looking for an answer [[12]].

What they found

In the Khon Kaen review, 76 of the 1,584 jaundiced newborns — about 1 in 21 — had hereditary elliptocytosis [[9]]. Their jaundice tended to start early — at around 38 hours after birth for the typical baby in the group. Most needed the standard blue-light treatment for around four days; seven needed a double set of lights; none needed the more intensive treatment called an exchange transfusion, in which a baby's blood is gradually replaced. Twenty-three of these babies later became anaemic — low in red blood cells — and 17 needed a blood transfusion, typically at about a month old and in one case as late as seven months. By six months, their blood counts had largely settled.

The Bangkok cord-blood study is the one that shows how often this is missed. Thirteen babies out of 1,505 carried one of the three gene changes, and nine of those thirteen needed phototherapy [[10]]. Only two of those nine were correctly diagnosed before going home. The other seven were sent home with a different label — early delivery, poor feeding, or simply "cause unclear." When the researchers accounted for everything else that raises jaundice risk, carrying one of these gene changes turned out to be the strongest single risk factor for needing phototherapy that they measured, making it about five times more likely. Their jaundice also started earlier than other babies', around 25 hours of age rather than 44. Most striking of all, most of these babies were not anaemic — so the usual clue that red cells are breaking down simply wasn't there.

In the Malaysian study, G6PD deficiency was the third most common reason babies were admitted for jaundice, after feeding-related causes, accounting for about 1 in 13 admissions [[11]]. Half of the G6PD-deficient babies became jaundiced in their first 24 hours, compared with about one in seven of the others. They needed the lights for longer — more than half needed over two days of treatment, compared with about one in five of the other babies — and were more likely to need an exchange transfusion. No baby in the study developed the serious brain injury that jaundice can cause. The same pattern of G6PD deficiency appears in newborn studies from Thailand [[13]] and, at higher rates, from South Kivu in the eastern Democratic Republic of the Congo, where about 6 in every 100 newborns tested were affected [[14]].

The Chinese study answered a different question: if you look hard, how often is there an inherited explanation? In 56 infants whose jaundice had no other explanation, genetic testing found one in 29 of them — just over half [[12]]. Some had Gilbert syndrome, a common and generally harmless variation that slows the liver's handling of bilirubin. Some had a rarer condition affecting how bile acids are transported, first described only in 2015 [[15]]. Ten of the infants had two inherited conditions at once, and those babies had the most stubborn jaundice of all.

What this means for families

The most useful takeaway is not alarming: none of these conditions, in these studies, caused brain injury, and almost all of the babies did well. What these studies show is that the explanation is often missing, and that the missing explanation occasionally matters weeks or months later.

If your baby had jaundice that started in the first day or two, lasted longer than expected, came back after treatment, or needed more light therapy than the staff anticipated — and no clear cause was found — it is reasonable to ask two questions. The first is whether a blood film was looked at under the microscope, which is the simplest test for a red-cell shape problem, and whether a targeted genetic test is available locally. The second, and arguably more important one, is whether your baby's haemoglobin — the standard blood test for anaemia — should be rechecked at four to six weeks. In the Thai study, the anaemia that needed treatment usually showed up at about a month of age, after most jaundice follow-up had already finished [[9]].

There is also a family dimension. Carrying one copy of these gene changes is usually harmless — many of the parents in these studies had no idea they carried anything. But when both parents carry one, a future child can inherit two copies and have a much more serious form of the condition that causes severe anaemia before or shortly after birth [[16]]. Knowing this is not a reason for worry so much as a reason for a conversation with a genetic counsellor before a next pregnancy.

What researchers are working on next

The open question is whether every newborn in high-prevalence regions should be tested for these gene changes, the way many are already tested for G6PD deficiency. The Bangkok researchers argue that testing cord blood — which is already being collected — would be better than waiting until a baby is visibly jaundiced, but they are careful to say that nobody has yet worked out whether it is worth the cost, currently about US$37 per test [[10]]. Researchers are also cautious about testing too broadly: a separate 2026 study of 1,019 newborns found that certain common gene variants linked to bilirubin caused only a mild, harmless difference by six weeks of age, and warned that reporting them routinely could worry families for very little benefit [[17]].

The practical work ahead is to follow more babies for longer — to learn how often the late anaemia happens, whether inheriting two of these conditions together is genuinely riskier, and whether any of it affects a child's development. Until those answers arrive, the sensible position is the one all four studies point toward: when a newborn's jaundice arrives early or overstays its welcome without explanation, an inherited cause is worth looking for, and it is entirely reasonable for a family to ask. Families wanting to understand the treatment itself may find an overview of how phototherapy intensity affects bilirubin clearance helpful ([In\[Neo\]Sight: does irradiance dose matter?](https://inneosight.org/en-US/articles/intensive-phototherapy2026)).

References

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