Are Baby Boys Born Early More Vulnerable? What New Research Says About Cerebral Palsy and Kidney Health

Two 2026 studies, a worldwide review of cerebral palsy in boys and girls and a look at kidney health in a large US trial of very premature babies, explain what the "boy disadvantage" really means for families

Boys born early are slightly more likely than girls to develop cerebral palsy, a lifelong condition affecting movement, and in babies born very early they are a little more likely to have kidney strain in the first weeks of life and higher blood pressure readings as toddlers. Two 2026 studies show these differences are real but small, and most boys born early do not develop cerebral palsy. The kidney researchers say their findings do not support monitoring boys differently because of their sex alone.

Why researchers keep asking about boys and girls

If your baby was born early, you may have heard a nurse or doctor say that boys "tend to have a harder time". That remark has a long history behind it. For many years, studies of premature babies have reported, almost in passing, that boys did a little worse than girls on several measures. In the United States, a large group of newborn units followed babies born before 28 weeks of pregnancy (babies born at 37 weeks or later count as full term) around the turn of the century. Among them, about 11 in every 100 boys and 7 in every 100 girls developed moderate or severe cerebral palsy [1]. In Australia, boys born very early were more likely than girls to die in hospital or to have a disability at 2–3 years [2]. In the UK, boys born at 23–28 weeks more often died or came to depend on extra oxygen (72 in 100 boys compared with 61 in 100 girls) [3]. When researchers pooled 41 studies of more than 600,000 babies, boys were more often affected by bleeding in the brain, long-term lung disease, a serious bowel disease and death [4]. Experts suspected a mix of causes, including hormones, genes and the immune system [5]. There was also some good news: in US newborn units, deaths and lung disease fell faster in boys than in girls between 2006 and 2016 [6]. Still, two big questions had not been answered. Does the extra risk for boys change depending on how early a baby is born, and has better care narrowed the gap? And does the boy disadvantage reach organs such as the kidneys, which had barely been studied in this way [7]?

The first study: combining 22 studies of cerebral palsy

Cerebral palsy is a group of conditions that affect movement and posture, caused by injury to, or differences in, the developing brain. The earlier a baby is born, the higher the chance: an earlier review found that about 15 in every 100 babies born at 22–27 weeks developed cerebral palsy, compared with 1 in every 1,000 born at full term [8].

A team at the University of Pavia in Italy carried out what is called a meta-analysis [9]. Instead of enrolling new babies, they gathered every suitable earlier study and combined the results, which makes the answer more reliable than any one study on its own. They found 22 studies, most of them using national or regional registries and cohorts, from Scandinavia, other European countries, North America, Japan, Korea and Australia. Together, these covered more than 6.5 million births and 15,734 children with cerebral palsy. The researchers compared boys and girls in three groups: babies born at 31 weeks or earlier, those born at 32–36 weeks, and those born at 37 weeks or later.

The second study: kidneys in babies born very early

The kidneys clean the blood and help control blood pressure. In babies born very early, the kidneys are still forming, because the tiny filtering units are mostly built in the last three months of pregnancy [10]. Sometimes they are strained in the first weeks of life, a problem called acute kidney injury, meaning a sudden drop in how well the kidneys work. It is common in the smallest babies: an earlier study found it in 38 of every 100 babies born at 24–27 weeks [11].

The second study was done separately from the first, by a different research group looking at a different part of the body. A team based at Indiana University, working with partners across the United States, took a second look at information already collected in the Preterm Erythropoietin Neuroprotection Trial (PENUT) [7]. This trial took place in 19 academic hospitals in the United States and tested whether a medicine called erythropoietin could protect the brains of babies born at 24 to 27 weeks. It did not, but it left behind detailed records [12]. A linked study, called REPAIReD (Recombinant Erythropoietin for Protection of Infant Renal Disease), checked the children's kidneys again at age 2 [13]. The new analysis did not test any treatment. It simply asked whether boys and girls differed, using records from 923 babies: 479 boys and 444 girls.

What the researchers found about cerebral palsy

In every group, boys were a little more likely than girls to develop cerebral palsy [9]. Overall, boys' risk was about 22% higher. That sounds large, but it is a percentage of a risk that is usually small. For babies born at 31 weeks or earlier, about 95 in every 1,000 boys and 81 in every 1,000 girls were later diagnosed with cerebral palsy. In other words, the great majority of boys in this group did not develop it. For babies born at full term, the numbers were about 1.5 in every 1,000 boys and 1.2 in every 1,000 girls.

Two further findings stood out. First, the boys' extra risk was about the same size whether a baby was born very early or at full term. Second, among the earliest babies, the gap seemed smaller in children born more recently. The researchers think better care during pregnancy and in newborn intensive care may have helped boys in particular. They are careful to say this needs to be confirmed with more detailed information before anyone relies on it.

What the researchers found about the kidneys and lungs

In the PENUT babies, kidney strain in hospital affected about 44 in every 100 boys and 37 in every 100 girls [7]. At age 2, a blood pressure reading above the usual range for a child's age and sex, which the researchers called high blood pressure, was found in about 66 in every 100 boys and 52 in every 100 girls tested. This was based on a single visit, and young children's blood pressure readings can vary a lot, so these numbers point to children who may need their blood pressure checked again rather than a confirmed diagnosis. The researchers used a cut-off from the American Academy of Pediatrics blood-pressure guideline [14].

Importantly, how well the kidneys filtered blood at age 2 was the same in boys and girls. Protein in the urine, another sign of kidney strain, was actually a little less common in boys, though that difference could have been chance. Boys also more often had serious long-term lung disease of prematurity (called bronchopulmonary dysplasia): about 33 in every 100 boys compared with 24 in every 100 girls. Kidney strain did not explain this lung difference. Death did not differ significantly between boys and girls in this group.

What this means for your family

The kidney researchers stress that the differences they found are modest and say plainly that their findings do not support monitoring babies differently based on sex alone [7]. Your baby's care in the neonatal unit will continue to depend mainly on how early they were born, how unwell they are and what treatments they need. The numbers above make the point: the difference between being born very early and being born at full term (about 95 compared with about 1.5 cases of cerebral palsy per 1,000 boys) is far larger than the difference between boys and girls [9].

There are some useful, practical points for families. If your child was born very early, it is sensible for their blood pressure to be checked at follow-up visits, whatever their sex, and a single high reading is a reason to measure again calmly rather than a cause for alarm. Our article on blood pressure in children born preterm explains more. Follow-up clinics look for early signs of cerebral palsy in both boys and girls, and early recognition helps children get support sooner. If you have a son, it is fine to ask your team how these findings apply to him; if you have a daughter, remember that girls can be affected too.

What these studies cannot tell us yet

It helps to know the limits. The cerebral palsy review only included studies from wealthier countries, and it could only count children who survived, since none of the studies reported deaths and cerebral palsy separately for boys and girls in the same group of babies [9]. The finding that the gap is shrinking comes from comparing studies rather than individual children, so it is less certain. Meanwhile, a US study found that cerebral palsy among surviving babies born before 27 weeks became more common between 2008 and 2019 [15], a reminder that a narrowing gap between boys and girls is not the same as fewer children affected. The kidney study included only babies born at 24 to 27 weeks, so it may not apply to babies born later, and an earlier look at the same group found no difference between boys and girls in kidney strain [16].

What researchers are working on next

The kidney team wants to understand why boys seem more vulnerable, because, they say, that knowledge could lead to treatments that help every baby. Researchers are calling for all newborn studies to report results separately for boys and girls, since many studies are too small to spot differences on their own [17]. Kidney specialists have made the same call [18]. A large Canadian study recently found that boys born early used more hospital and doctor services in early childhood, however early they were born, and called for follow-up care that takes sex into account [19]. The cerebral palsy team says the shrinking gap still needs to be confirmed with more detailed data on individual children, and following children's blood pressure and kidneys for longer would show whether the early differences last.

The bottom line

Boys born early face a small extra risk of cerebral palsy, of kidney strain and lung disease as newborns, and of higher blood pressure readings as toddlers, but most boys born early do not develop cerebral palsy, and how early a baby is born matters far more for cerebral palsy. For now, the best response is good, careful follow-up for every child born early, boy or girl.

References

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