Pregnant women who take the recommended folic acid supplements may be more likely to have twins, say researchers in Sweden, the New Scientist reports this week. Bengt Källén of the Tornblad Institute in Lund and his team looked at records from the Swedish Medical Birth Registry, which has gathered data on the use of medicines by pregnant women since 1994. According to the records, 2569 women said they used folic acid supplements. The rate of twin births for these women was 2.8 per cent. In the general Swedish population, however, 1.5 per cent of births in the late 1990s were twins. The reasons for the difference are not clear. Källén's team tried to allow for other factors - older mothers and those using IVF, for instance, who are already more likely to have twins. It is possible that folic acid somehow increases the probability of multiple ovulation or implantation of more than one egg. Källén says more research is needed to pin down the effects of folic acid tablets. "I think one should consider the pros and cons - especially in areas like Sweden with a low rate of spina bifida," he says. In the US the compulsory addition of folic acid to breakfast cereals and wholemeal bread since 1998 has reduced the incidence of spina bifida by 19 per cent. This has increased pressure on other nations to follow suit but this new Swedish study implies that each country should carefully the positives and negatives to introducing such a measure in particular in countries where spina bifida is relatively uncommon.