Blood Clots in Newborn Babies: How Common They Are, Which Babies Are Most at Risk, and Why Many Clear Within Weeks

What two large 2026 research summaries by Pelland-Marcotte and colleagues and a 20-year hospital study by Yan and colleagues tell families about clots in babies' veins

Babies in intensive care sometimes develop blood clots in their veins, most often around the thin tubes doctors use to give nutrition and medicines. New research shows these clots are more common in babies with heart problems or infections, but also that many clots shrink or disappear within a few weeks of treatment, which may mean some babies can stop blood-thinning injections sooner.

Why clots in newborn babies matter

A blood clot is a clump of blood that has turned from liquid to gel. Clots are the body's way of stopping bleeding, but sometimes one forms inside a blood vessel where it is not needed. When this happens in a vein, the vessels that carry blood back to the heart, doctors call it venous thromboembolism, often shortened to VTE, or deep vein thrombosis when the clot is in a large vein of an arm, leg or the chest. In newborn babies, clots most often form around a central line, a soft, thin tube threaded into a large vein so that a baby can receive feeds and medicines directly into the bloodstream. Central lines are often life-saving, but the tube itself can disturb blood flow and irritate the vein wall.

How doctors used to think about newborn clots

The three new studies described below [1][2][3] build on a long history. Thirty years ago, clots in newborns were diagnosed only rarely. In the early 1990s, doctors at McMaster University in Canada set up an international register to collect cases and gathered 97 babies from 29 hospitals. Apart from one kind of clot in the kidney vein that appears on its own soon after birth, almost 9 in 10 of those clots were linked to a tube in a blood vessel, and treatment varied widely from hospital to hospital because there was so little research to guide it [4]. A Canadian register for older babies and children, from 1 month to 18 years, found that infants under 1 year were among the groups most often affected [5]. Over the following decades, the number of clots diagnosed in children's hospitals in the United States rose sharply, including in newborns [6], and kept rising until at least 2019 [7]. Even so, large studies of neonatal intensive care units (NICUs) gave very different answers, from about 2 clots in every 1,000 babies in one United States network [8] to about 15 in every 1,000 in Canadian units [9]. Doctors also had little evidence about how long to treat: the experts who wrote the guidance on blood thinners for babies and children acknowledged that the evidence behind most of their advice was weak [10]. Babies' blood clotting system is still developing, and the length of treatment has varied widely between countries [11].

Three studies that answer three questions in turn

In 2026, three studies tackled the questions families most often ask. A team led by Dr Pelland-Marcotte, with colleagues in Quebec City (Canada), Rotterdam (the Netherlands) and Chicago (United States), carried out two linked "meta-analyses". A meta-analysis gathers all the earlier studies on a question and combines their results to get a bigger, steadier picture. Their first meta-analysis asked how often clots happen and combined 82 studies covering about 3.5 million newborns [1]. Their second asked which babies are most at risk and combined 60 studies of about 3.4 million newborns [2]. A separate team at the Stollery Children's Hospital in Edmonton, Canada, then looked back over more than 20 years of their own patients to ask how quickly clots go away once treatment starts [3].

How often clots happen, and why the number needs care

When the first meta-analysis averaged all the studies together, about 11 in every 100 newborns (10.6%) in those studies had a clot in a vein [1]. That figure sounds alarming, but it needs careful reading. The studies were very different from one another, many were small, and smaller studies tended to report higher rates, so the true figure may be lower. Studies that went looking for clots with routine ultrasound scans, a painless picture made with sound waves, found more of them (about 15 in 100) than studies that waited for signs of a problem (about 8 in 100). The large hospital databases mentioned above, which count clots that doctors actually diagnosed, found between 2 and 15 in every 1,000 babies. Rates were highest in babies with heart disease (about 17 in 100 in those studies) and for clots in the main vein of the liver, called the portal vein. In children, clots found only by routine scans are mostly temporary and usually do well [12], which is why the researchers caution that routine scanning could lead to babies being treated for clots that would never have caused harm [1].

Which babies are most at risk

The second meta-analysis found four things linked to a higher chance of a clot [2]. Babies whose mothers had preeclampsia, a condition of high blood pressure in pregnancy, had about two and a half times the odds. Babies with a low birth weight had about twice the odds, babies with an infection about three times, and babies with heart disease the highest odds, though that estimate was the least precise. Being born early was not clearly linked once all the studies were combined, even though some single studies had suggested it was [13], and neither was being a boy or a girl. The researchers rated their confidence in all of these findings as very low, because most of the original studies had important weaknesses. These are warning signs that help doctors and nurses stay alert, not predictions about any one baby.

How quickly clots go away

The Edmonton study followed 151 babies aged 3 months or younger, including 79 newborns, who had 179 clots in the deep veins of the arms, legs or large central veins between 2003 and 2025 [3]. Most were treated with injections of a blood thinner called low-molecular-weight heparin, with blood tests to check the dose was right. The hospital repeated an ultrasound scan 2 to 4 weeks after the clot was found, and if the clot had gone, the injections were stopped.

About 1 in 6 clots had completely gone by 2 weeks, more than 1 in 4 by 4 weeks, and just under half by 3 months. Overall, just over half of the clots disappeared completely during follow-up, and a further 1 in 5 got smaller. Clots in the arm or neck, and clots that only partly blocked the vein, tended to clear faster. Only 6 babies in the study had a new clot within a year, and in 5 of them the new clot was in a leg, so doctors may be more cautious about stopping treatment early for leg clots. The way young children's clots are built may also help: laboratory studies in healthy infants and children show that their clots have a looser structure, which may make them easier for the body to break down [14].

What this means for your baby

If your baby has a central line, it helps to know that clots are a recognised and closely watched complication, not a sign that something was done wrong. Nurses check lines and limbs carefully, and a line that stops working well or a swollen arm or leg is a reason for a scan. A large study linked lines left in place for 3 weeks or more to clots in veins [15], one reason teams take lines out once they are no longer needed. If your baby is treated for a clot, it is reasonable to ask whether a follow-up scan is planned, because, as the Edmonton team suggests, a scan at 2 to 4 weeks may show the clot has gone and allow treatment to stop sooner. Shorter treatment matters because blood thinners carry a risk of bleeding: in a Dutch study of 115 newborns with line-related clots, about 8 in 100 had a serious bleed [16]. A large international trial in children and young people at 42 hospitals in 5 countries also found that 6 weeks of treatment worked as well as 3 months when a scan at 6 weeks showed the clot had gone or no longer fully blocked the vein, though very few newborns took part [17].

For a wider look at how doctors recognise and treat clots in newborns, including blood thinners given by mouth, see our earlier article on neonatal thromboembolic disease.

What researchers are working on next

These studies have limits. The Edmonton study looked back at records from one hospital where many babies had heart surgery, and it could not reliably record bleeding, so it cannot yet prove that stopping early is safer [3]. Experts writing about the study, who also worked on the two meta-analyses, called it potentially practice-changing, while noting that it looked back at records from a single hospital and that some scans were done later than planned [18]. Researchers now want studies that follow babies forward in time, scan them on a fixed schedule, record bleeding carefully, and build tools to predict which babies are most likely to develop a clot.

The bottom line

Clots in veins are a real and fairly common complication for the sickest newborns, especially those with heart disease, infection, low birth weight or a mother who had preeclampsia, although the true rate in a typical intensive care unit is lower than the 11 in 100 headline figure. The encouraging news is that many clots clear within weeks, and a planned follow-up scan may help some babies finish their blood-thinning injections sooner.

References

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  2. Pelland-Marcotte MC, Pérez Herrera NM, Boileau E, van Ommen H, Bhat R. Factors Associated With Thromboembolism in Neonates: A Systematic Review and Meta-Analysis. JAMA Netw Open. 2026;9(5):e2610908. doi:10.1001/jamanetworkopen.2026.10908 ↩
  3. Yan E, Bauman M, Meakins L, Bruce A, Livingston J. Early venous thrombus resolution in neonates/infants: retrospective review of a 20-year single institution experience. Res Pract Thromb Haemost. 2026;10(3):103455. doi:10.1016/j.rpth.2026.103455 ↩
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  7. O'Brien SH, Stanek JR, Witmer CM, Raffini L. The Continued Rise of Venous Thromboembolism Across US Children's Hospitals. Pediatrics. 2022;149(3):e2021054649. doi:10.1542/peds.2021-054649 ↩
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