A diet high in dairy products can greatly increase a woman's chances of having twins, research suggests.
A protein found in animals' livers may be the cause
A study in the Journal of Reproductive Medicine showed milk drinkers were five times more likely to have twins than women who ate no animal products.
The numbers of twins in the world has increased significantly in the past 30 years, in some countries by over 50%.
Scientists have suggested fertility treatments and women delaying pregnancy can help explain the rise.
But this new research indicates that diet can also play an important part.
In the study, the twinning rates of women who ate a diet including milk were compared with women who followed a vegan, or no animal products diet.
It is believed that a protein found in the livers of animals may be the cause. Called Insulin-like Growth Factor or IGF, it is found in cow's milk and other animal products.
In women it makes the ovaries more sensitive and increases the number of eggs produced. Higher levels of IGF improve the survival chances of an embryo in the early stages of development.
The effect is likely to be greater in countries such as the United States that allow growth hormones to be fed to cattle.
The researcher behind this study says that women thinking of getting pregnant might consider alternatives to meat and dairy products to reduce their chances of having twins, as multiple births are more prone to complications.