Vegetables such as broccoli are a good source of folic acid

Folic acid is already known to reduce the risk of brain damage and spina bifida, and it was recently suggested it could also cut the risk of a baby being affected by Down's syndrome.

A deficiency in folate levels has been linked to damage to DNA, and to an increased risk of developing some adult cancers, including breast and colon.

Scientists also know that many cases of childhood cancer are influenced by chromosomal damage before birth.

Genetic differences

Researchers from the Institute of Cancer Research, looked at whether the folic acid levels the child was exposed to in the womb influenced their chance of developing leukaemia.

The realisation that damage to the DNA in these cells can be reduced with folate supplements in pregnancy is extremely exciting Dr David Grant, Leukaemia Research Fund

They focussed on an enzyme called MTHFR which breaks down folic acid and reduces its levels.

But some people inherit a variant of MTHFR gene which makes the enzyme inactive, so they have higher levels of folic acid because their bodies cannot break it down.

Scientists studied 253 British children who were diagnosed with leukaemia from 1992 until 1998 and compared them with healthy children.

It was found that those who inherited the inactive enzyme had a significantly lower risk of leukaemia compared with those with a normal MTHFR gene.

DNA damage

Dr David Grant, science director of the Leukaemia Research Fund which funded the research, said it showed some children who developed leukaemia were "almost certainly born with defective blood cells which go on to become leukaemic cells."

He added: "The realisation that damage to the DNA in these cells can be reduced with folate supplements in pregnancy is extremely exciting and could help to reduce the number of cases of this terrible disease in children."

The study was carried out as part of the UK Childhood Cancer Study.

A second study from Australia found if mothers took folic acid during pregnancy, it almost halved the risk of their children of developing leukaemia.

Women are currently advised to take folic acid supplements while trying to conceive, and in the first three months of pregnancy.