Taking folic acid supplements late into pregnancy may increase a mother's risk of breast cancer, research suggests.
The breast cancer findings are only preliminary, stress the researchers
However, the finding in no way questions the benefit of taking the supplements before and during the first months of pregnancy.
Folic acid taken at this time is known to reduce the risk of neural tube defects, such as spina bifida.
The research, by the Universities of Aberdeen and Bristol, is published in the British Medical Journal.
The scientists say this may be a chance finding, and stress the results require confirmation by follow-up studies.
Department of Health recommendations
Prospective mothers should take 0.4mg per day of folic acid before conception.
This should continue to the 12th week of pregnancy.
Until now, there has been little evidence on the long-term effects of increased folic acid intake during pregnancy.
The researchers focused on 2,928 pregnant women who took part in a trial of folic acid supplementation in the 1960s.
The women were given either a 0.2mg or 5mg dose of folic acid, or a dummy pill.
By the end of September 2002, 210 women had died with 40 deaths attributable to cardiovascular disease, 112 to cancer and 31 to breast cancer.
The overall death rate in women who took the high dose supplements was around 20% higher - and they were twice as likely to have died from breast cancer.
No need for confusion
Researcher Dr Andy Ness said: "Our paper presents preliminary findings which are intended to point the way towards further research and it is published on that basis.
"It is entirely possible that this is a chance finding - so further scientific studies are required to examine the association, if there is one, before we reach any conclusions."
Dr Ness stressed it was important not to confuse women about the benefits of taking folic acid supplements in early pregnancy.
"Women planning to become pregnant should take folic acid supplements as recommended," he said.
Professor Marion Hall, of the University of Aberdeen, said: "This was an unexpected but also a very preliminary finding and further research will be required to ascertain whether this is a "true" finding.
"No pregnant woman needs to change their current practice with folic acid but the study suggests that our findings should be taken into account when policies are being developed for advising pregnant women to take anything during pregnancy and at other times."
Andrew Russell, of the Association for Spina Bifida and Hydrocephalus, said previous research had found that higher levels of folic acid in the blood actually lowered the risk of breast cancer.
He said the latest findings could probably be explained by chance, and stressed that women were at greater risk if they decided to abort a foetus following a diagnosis of spina bifida.
Many campaigners are calling for folic acid to be added to wheat and corn flour products as a matter of course.
Mr Russell cited a Canadian study which found the proportion of babies born with neural tube defects dropped by 78% after the compulsory addition of folic acid to flour cornmeal and pasta in 1998.
Antonia Bunnin, of the charity Breakthrough Breast Cancer, said: "These are preliminary findings which should be treated with caution - folate has known health benefits and other studies have suggested that it may help prevent breast cancer in some women.
"More research is needed into the long-term effects of folate intake on breast cancer risk before firm conclusions can be made."
Helen Brambhatt, a nurse consultant for the charity Breast Cancer Care, said women should not be concerned as the study had produced only preliminary findings.
She agreed that further research was needed to provide conclusive findings.
Rosemary Dodds of the National Childbirth Trust echoed the advice, but added: "If women are concerned, they can stop taking their supplements later on in their pregnancy and continue to eat foods rich in folic acid, such as green leafy vegetables, citrus fruits and fortified cereals."