The New York Times reports mothers who regularly consume dairy are fives times more like to have fraternal twins than those who do not. Experts point to hormones given to diary cows as explanation. Nicholas Bakalar reports:
Dr. Gary Steinman, an assistant clinical professor of obstetrics at the Albert Einstein College of Medicine, reached that conclusion by looking at the medical records of 1,042 mothers who were vegans consuming no dairy products and comparing them with those of mothers who regularly ate dairy products.
His findings appear in the May issue of The Journal of Reproductive Medicine. Eating dairy products increases blood levels of insulinlike growth hormone, or I.G.F., and it is this increased hormone level that is associated with increased rates of multiple ovulation.
In a study published in 2000 and cited in the findings, vegan women had concentrations of I.G.F. that were 13 percent lower than those in women who regularly consumed dairy products...
The article also discusses the potential role of synthetic hormones given to cows:
Many dairy farmers inject their cattle with recombinant bovine somatotropin, a synthetic version of the naturally occurring hormone. This increases size and milk production, but it has another effect: cows with higher growth hormone levels produce more twins.