Women who take a daily multivitamin tablet can boost their chances of getting pregnant.

Researchers at the Harvard School of Public Health who followed 18,000 women over eight years found those who took a multivitamin on at least six days a week had a 40 per cent lower risk of suffering ovulatory problems leading to infertility.

About 150,000 women in the UK have ovulation problems - almost one in 10 of all women affected by infertility. It is the second leading cause of female infertility after problems with blocked fallopian tubes.

Jorge Chavarro, the research fellow at Harvard who led the study, said it was the largest so far conducted. The women had all successfully become pregnant or had tried and failed to do so, and were selected from a separate research project which has followed the health of 100,000 nurses over 15 years.

"Multivitamins are packed with a large amount of nutrients, so we looked at which were crucial. As best as we could tell it looked as if folic acid was the nutrient that played the biggest role," Dr Chavarro said.

A lack of folic acid, which is found in leafy green vegetables and liver, is linked with spina bifida. Pregnant women and those trying to conceive are advised to take supplements.