Women who eat a vegan diet — a strict vegetarian diet that excludes all animal products including milk — are one-fifth as likely as other women to have twins, a U.S. researcher reported.
The reason may be hormones given to cattle to boost their milk and meat production, said Dr. Gary Steinman, an obstetrician specializing in multiple-birth pregnancies at Long Island Jewish Medical Center in New Hyde Park, New York.
Writing in the Journal of Reproductive Medicine, Steinman said he compared twin births rates among women who ate a regular diet, vegetarians who included dairy products, and vegan women.
The vegans had twins at one-fifth the rate of the milk-drinking women. Insulin-like growth factor may be responsible, Steinman said.
All animals, including people, produce a compound called insulin-like growth factor or IGF in response to growth hormone. It is found in milk and it increases the sensitivity of the ovaries to follicle stimulating hormone, thus increasing ovulation.
Some studies also suggest that IGF may help embryos survive in the early stages of development.
Vegan women have about a 13 percent lower level of IGF in the blood than women who consume dairy.
The number of multiple births, including twins, has increased significantly in the United States since 1975, about the time assisted reproductive technologies were introduced.
And women are waiting until they are older to have children, which can increase the rate of twin boths.
But Steinman thinks something else may be contributing to the increase in the rate of twin births.
"The continuing increase in the twinning rate into the 1990s, however, may also be a consequence of the introduction of growth-hormone treatment of cows to enhance their milk and beef production," he said.
There could be a genetic link to IGF's influence, also, Steinman said.
In cattle, regions of the genetic code that control the rate of twinning have been found close to the IGF gene.
And black women in the U.S. have, on average, the highest rates of twin births — and they also tend to have normally higher levels of IGF in their blood, Steinman said.
Asian women have the lowest IGF levels and the lowest rate of twin births and Caucasian women fall in-between, he said.
"Because multiple gestations are more prone to complications such as premature delivery, congenital defects and pregnancy-induced hypertension in the mother than singleton pregnancies, the findings of this study suggest that women contemplating pregnancy might consider substituting meat and dairy products with other protein sources, especially in countries that allow growth hormone administration to cattle," said Steinman.