FOR decades, doctors have blamed the rise in the number of women who give birth to twins entirely on the widespread use of fertility treatment. But now they have found a much more natural cause — a glass of milk.

Scientists have discovered that drinking milk and eating cheese stimulates a protein than prompts the release of eggs and makes women five times more likely to give birth to twins.

The findings have been made by researchers from the Long Island Jewish Medical Center in New York who wanted to know why the rate of twin births in the United States rose by more than 75% between 1980 and 2003.

Some of the increase can be explained by the wider use of fertility treatments, which increase the chance of multiple births, but they doubted this could account for the entire rise.

They suspected changes in diet may have contributed to the change, and compared the records of more than 1,000 vegan women with mothers who ate animal products. They found that the twinning rate in vegans is one fifth of that in vegetarians and meat-eaters.

The research, published in the Journal of Reproductive Medicine, says eating animal products, in particular dairy foods, boosts the production of a protein called insulin-like growth factor (IGF) in women. The scientists say the protein promotes the release of eggs by the ovary, increasing the chance of two or more eggs being fertilised at the same time.

In America there has also been an increased use of growth hormones in dairy cows. The growth hormone is banned in British cattle.

Although Dr Gary Steinman, the lead scientist, admits his findings are preliminary, he points to other research that backs his theory. Research has shown that vegan women’s levels of the IGF protein are lower than those of other women while separate work has suggested that the rate of twin production is higher in countries that consume more milk.

“During periods of malnutrition — for instance during the second world war — the twinning rate decreased, perhaps because of a fall in maternal concentrations of the insulin-like growth factor,” said Steinman.

Other scientists say vegan women may bear fewer twins because they are less well nourished. Dr Paul Haggarty of the Rowett Research Institute in Aberdeen says there may be other nutrients that vegan women lack.

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