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.

美国一项最新研究显示，那些常常食用奶制品的女性，生双胞胎的可能性是那些不常食用奶制品的女性的5倍。研究人员指出，造成这一现象的原因可能与人们喂养奶牛时添加的生长激素有关。

据生活网日前报道，此项研究的负责人、美国长岛犹太医疗中心的加里斯坦曼说，摄入生长激素后，人和动物的肝脏能分泌一种名为IGF的生长蛋白。

而IGF会随着血液循环进入动物的乳汁。IGF能增加排卵作用同时还可能有助于怀孕早期阶段胚胎的存活。

研究显示，那些秉承绝对素食主义的妇女血液中IGF的浓度要比那些常食用奶制品的妇女低大约13％。绝对素食者拒绝食用一切源于动物的食物。自1975年以来，生育多胞胎的比率明显上升。

其中一部分原因要归咎于更多的大龄妇女怀孕，因为统计数据显示，大龄妇女更易生育双胞胎。然而，生育双胞胎的比率在上世纪90年代仍持续增长则可能是由于人们为了给奶牛催奶或增加牛肉产量使用生长激素的结果。