The scientists studied the cases of 2,569 women who told the Swedish Medical Birth Registry they
The rate of twin births among these women was 2.8 per cent - almost twice that in the general population, where 1.5 per cent of births were twins.
Dr Kallen calculated that, if 30 per cent of 100,000 women in Sweden took folic acid supplements, there would be 225 extra pairs of twins.
These 450 babies would often be premature, have low birth weight and at higher risk of cerebral palsy.
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Dr Kallen is not sure how folic acid might increase the chances of multiple births.
But his report - published today in the magazine New Scientist and the journal Twin Research - says it is possible the vitamin somehow increases the probability of multiple ovulation or implantation of more than one egg.
Sir John Grimley Evans of Oxford University, who chaired a committee set up by the government to investigate whether folic acid should be added to flour, said it was important that any risks from extra folic acid were identified.
He has informed the Health Department of the Swedish report.