Women who eat animal products, and specifically dairy, are five times more likely to have twins if they become pregnant, says a new US study.
Gary Steinman, an obstetrician and specialist in multiple-birth pregnancies, found the results by comparing the twinning rates of pregnant women on vegan diets and those who ate animal products.
The study, to be published in this month's Journal of Reproductive Medicine, adds to scientists' understanding of how diet may influence pregnancy.
"This study shows for the first time that the chance of having twins is affected by both heredity and environment, or in other words, by both nature and nurture," said Steinman, of the Long Island Jewish Medical Center in New York.
The study suggests a protein known as insulin-like growth factor (IGF) may be responsible for the increased twinning rate. It says the amount of IGF in a woman's blood may be directly proportionate to her chance of having twins.
IGF is released from the liver of both animals and humans and is found in animal's milk. Previous studies have indicated IGF may increase ovulation and even help embryos survive in the early stages of their development.
Women who consume dairy have 13 per cent more IGF in their blood on average than those on a vegan diet.
Production of the IGF protein is triggered by the presence of growth hormones. This has led Steinman to suggest the introduction of growth-hormone treatment for cows in the 1990s, to enhance milk and beef production, may have contributed to a rise in the number of twins over the last decade.
The lifestyle trend towards later child birth is also thought to contribute, as older women are more likely to have twins.
Steinman, however, believes IGF levels are significant, and used his study to wade into the fiery debate across the UK and US on the pros and cons of eating dairy.
He said since multiple births held more health risks for women, and "those contemplating pregnancy might consider substituting meat and dairy products with other protein sources".