Down's syndrome is the most common genetic disorder in babies. Two babies a day are born with the extra chromosome, which causes learning difficulties and physical abnormalities.
Although the risk of giving birth to a Down's syndrome child increases beyond the age of 35 to one in 350, most babies with Down's syndrome are born to mothers under 30.
Scientists at the US Food and Drug Administration (FDA), who published the study in the American Journal of Clinical Nutrition, said the results were a promising step towards a better understanding of Down's syndrome.
But they said it was too soon to tell whether adding folic acid to women's diets could improve metabolism and cut the rate of Down's syndrome, as with some other birth defects.
"Our preliminary research is promising. However, a larger clinical study is needed," said Jill James, a biochemist who conducted the research at the FDA's National Center for Toxicological Research.
The study compared 57 mothers of children with Down's syndrome to 50 mothers of children without. The mothers' average age was 30.
Scientists found that more of the mothers of Down's babies had a genetic change in an enzyme involved in the metabolism of folic acid. It is already known that adding folic acid to diets normalises the body's metabolism of the nutrient and significantly reduces the occurrence of birth defects known as neural tube defects.
Doctors recommend that women take folic acid supplements prior to conception and in the first three months of pregnancy to reduce chances of having a child with spina bifida. Last year the FDA instructed manufacturers to fortify flour products with folic acid to cut the rate of neural tube defects, including spina bifida.
Folic acid is found naturally in green leafy vegetables and other foods. Research has shown that increasing folic acid levels before conception lowered neural tube defects, which are serious and often fatal, by 50 to 70 per cent.
Now FDA researchers are studying whether extra folic acid intake would reduce the risk of Down's syndrome, Dr James said. Researchers needed to show whether folic acid would normalise this. "It works for neural tube defects, so it is worth checking," she said.
A spokesman for the Down's Syndrome Association said: "There is nothing available to predict whether mothers will have a Down's syndrome child at the moment, but this research is highly speculative."