Half the population carry gene variations which increase the risk of birth defects such as spina bifida, research has found.
Folic Acid can minimise the risk
Scientists say the finding underlines the importance of women taking folic acid supplements to reduce the risk of neural tube defects in their babies.
The research, by Irish and US teams, is based on analysis of 395 people born with neural tube defects.
It is published in the British Medical Journal.
Folic acid, when broken down in the body to folate, plays a key role in ensuring that the spinal column - or neural tube - develops normally during pregnancy.
Thus low levels of folate increase the risk that the neural tube will fail to close up properly - leading to spina bifida, or other related conditions.
Scientists already knew that one particular variant of the gene responsible for processing folate - the TT variant - is associated with an increased risk of neural tube defects.
People who carry this form of the gene have lower levels of folate in their tissues. They also have higher levels of another compound called homocysteine which scientists suspect also plays a role in the formation of the nervous system.
The latest research has shown that another form of the gene - the CT variant - can lead to the same problem.
The analysis found that the CT variant was responsible for at least as many neural tube defects in the population as the TT variant.
This is because a much greater proportion of the general population (38%) are CT compared with 10% who are TT.
The researchers have calculated that combined the two variants account for 26% of neural tube defects in Ireland.
Researcher Dr Peadar Kirke, of Dublin's Health Research Board, told BBC News Online: "This is a rare situation where there is a potential problem during pregnancy, but it can easily be corrected.
"I would advise any woman who is thinking of having a baby to ensure that she takes 400mg of folic acid a day for at least a month before she becomes pregnant."
Dr Kirke said the research also suggested that it would be a good idea to fortify flour products with folic acid - an idea which has so far failed to be adopted in any European country.
He said: "Forty countries around the world have either fortified flour products, or have plans to do so, but none of these are in Europe.
'Europe lagging behind'
"I feel very strongly that Europe is really lagging behind in a very serious negative way in not fortifying food."
Andrew Russell, chief executive of the Association for Spina Bifida and Hydrocephalus, said: "We are not doing enough to inform the public about the need to take folic acid, or to fortify the diet, and any evidence, such as this, that underlines the need to do that is very welcome."
Around one baby in 1,000 is born with a neural tube defect. Many are terminated during pregnancy.
It is estimated that taking folic acid supplements before pregnancy would prevent up to 80% of cases.
But while folate is known to be a key factor in the prevention of neural tube defects, others factors also probably play a role.
Mr Russell said: "It may be a complex interaction of environmental and genetic factors."