The risks are still small, researchers say, as children only rarely develop brain cancer. Each year, about 4000 U.S. children and teenagers are diagnosed with a tumour of the central nervous system (brain or spinal cord), according to the American Cancer Society.
A small portion are caused by specific, inherited genetic syndromes, but otherwise little is known about why children develop brain and spinal cancers.
The new findings, published in the journal Paediatrics, highlight the potential importance of genetic factors, the researchers say.
Using a California database on cancer cases in the state, the researchers found 3,733 cases of brain or spinal tumours diagnosed among children younger than 15 between 1988 and 2006.
Overall, 1.2% of those children had been born with a birth defect - versus 0.6% of 15,000 cancer-free California children studied for comparison.
And children with a birth defect had increased risks of certain tumours.
They were nearly 4 times as likely as children without birth defects to develop a primitive neuroectodermal tumour - tumours that start in immature cells of the brain, and account for about one-fifth of childhood brain tumours.
Similarly, their risk of germ cell tumours, a rare form of brain tumour in children, was elevated more than 6-fold.
Children with birth defects were not, however, at higher risk for the most common type of brain cancer in the study group - gliomas, which accounted for 57% of cases.
The study also found heightened tumour risks among children whose mothers had had at least 2 late pregnancy losses in the past - meaning the foetus died after the 20th week of pregnancy, commonly known as a stillbirth.
These children were about 3 times as likely as other kids to develop some type of brain or spinal tumour.
Since both birth defects and pregnancy losses often involve some type of genetic abnormality, it's possible that that explains the higher cancer risks, according to the researchers.
"Genetics may play a larger role in central nervous system cancer than previously believed," said lead researcher Dr. Sonia Partap, of Stanford University and Lucile Packard Children's Hospital in Palo Alto, California.
Miscarriages were not linked to cancer risks in a woman's other children. So it's possible that the genetic abnormalities that cause early pregnancy loss are not connected to cancer, while gene defects that are "compatible with life to some degree" do contribute to cancer risk, Partap told Reuters Health in an email.
As for birth defects, past studies have connected them to higher risks of childhood cancers in general.
But researchers are still trying to figure out whether it's only certain birth defects that come with a higher risk. Some preliminary evidence, Partap said, suggests that defects of the heart and brain may be particularly linked to childhood cancer.
But Partap also stressed that even with a relatively increased risk of brain or spinal cancer, the absolute risk to any one child is small.
"Parents should know that there is still a very low risk of central nervous system cancer," Partap said.
At the same time, she added, paediatricians should be aware that there is a slightly higher chance of the tumours in certain children.
Symptoms of brain tumours may be vague and vary from child to child. But some possible signs include morning headaches; mental changes like memory and concentration problems; unusual sleepiness; changes in vision, hearing or speech; and balance or coordination problems.
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