Adding folic acid to flour could see more women giving birth to twins after IVF treatment, a study suggests.
Folic acid might be added to white bread
UK food experts are considering mandatory fortification as folic acid can help cut the risk of birth defects.
Aberdeen University research published in The Lancet found twin births rose with higher folate intake, when more than one embryo was transferred.
But no link between folate levels and the likelihood of a successful pregnancy was established.
Folic acid, a synthetic form of vitamin B, cuts the risk of neural tube defects such as spina bifida.
In the UK, it is recommended that women take 400 micrograms a day up to week 12 of pregnancy.
The Food Standards Agency (FSA) favours adding folic acid to the nutrient mix already added to white flour.
Under draft proposals the vitamin would not be added to wholemeal products, or speciality breads.
The study by Dr Paul Haggarty and colleagues from the Rowett Research Institute and Department of Obstetrics and Gynaecology at Aberdeen University involved 602 women undergoing IVF treatment.
The women's diets - including their folate intake - was assessed, and their blood concentrations of folates were measured.
A high level of folic acid in the blood was linked to an increased chance of giving birth to twins.
But it did not increase the chance of a successful pregnancy.
The researchers said the findings were consistent with an 11-13% increase in multiple births in the US following fertility treatments after flour was fortified with folic acid in 1988.
Similar levels of flour fortification in the UK would result in 600 extra women a year having twins after fertility treatment, the researchers said.
To increase the chance of a pregnancy, two embryos are often transferred to a woman during IVF treatment.
Multiple births, however, can be associated with birth defects, such as blindness and cerebral palsy, and often predict poor survival.
The study also suggested there was a connection between a key gene involved in folate metabolism and a successful pregnancy.
Dr Haggarty said: "The high incidence of twin births associated with treatment for infertility could be reduced, while maintaining live birth rates, encouraging women not to exceed recommended doses of folic acid."
Professor William Ledger, from the Royal College of Obstetricians and Gynaecologists, said: "If folic acid supplementation improves pregnancy rates after IVF then this could be used to improve the outcome of single embryo transfer.
"Sadly, even in 2006 the outcome of a twin pregnancy is not as good as for a singleton.
"There should be careful attention paid to the public health message that folic acid supplementation for women trying to conceive is a good thing, since it substantially reduces the risk of spina bifida in the child, but that you can get too much of a good thing."
The Food Standards Agency said any decision on adding folic to flour will take account of advice from Scientific Advisory Committee on Nutrition, as well other opinion.
"A watching brief will be kept for emerging evidence in this area," it added.