Taking folic acid during pregnancy may reduce the risk of having a baby with cleft lip and palate.
Vegetables such as broccoli are a good source of folic acid
Researchers say extra supplements may be helpful for women who have a family history of the condition.
It has long been known that folic acid reduces the risk of spina bifida and brain damage in babies.
More recently, there have been suggestions that it may also lower the risk of Down's Syndrome and leukaemia.
Women are currently advised to take folic acid supplements while trying to conceive and also in the first three months of pregnancy.
"Our research has shown that there may be a susceptible group of women where taking additional folic acid early in pregnancy may lower the risk of having a child with cleft lip and palate," said lead researcher Professor Winter of the Institute of Child Health, London.
"Further studies are needed to confirm our findings, but our results are very encouraging."
Cleft lip and palate is one of the most common birth malformations, affecting at least one in every 1,000 babies born in the UK.
It occurs in the early weeks of pregnancy when the bones in the face and roof of the mouth fail to knit properly.
Children have to go through a number of operations, as well as speech therapy and treatment,to correct the disorder.
l hope there will now be clearer guidelines on which groups will benefit most from taking
folic acid before, and during early pregnancy
Gareth Davies, Cleft Lip and Palate Association
There is a strong genetic link - a baby is 30-40 times more likely to develop cleft lip and palate if their elder brother or sister was also born with the condition.
It appears that a particular group of mothers may be susceptible to having a child with a cleft.
They have a change in a gene called Methylenetetrahydofolate Reductase, which alters the way folic acid is processed in the body.
Gareth Davies, Chief executive of the Cleft Lip and Palate Association, said past studies had indicated that taking folic acid may help prevent cleft lip and palate in so called "high risk" groups but until now there had been no conclusive evidence to back this up.
"We welcome this research and l hope there will now be clearer guidelines on which groups will benefit most from taking
folic acid before, and during early pregnancy," he said.
The latest study was funded by Action Research. The medical charity also supported the original research that discovered the link between folic acid and the prevention of spina bifida.
The research was published in the Journal of Medical Genetics.