Accoording to a report published by Scientific American, milk and meat from cows raised on Bovine Growth Hormone contribute to women having more twins:

Over the last 30 years, the number of twin births has nearly trebled. This rise seems to have followed the introduction of in vitro fertilization and a preference for having children later in life. But in the mid-1990s, doctors began limiting the number of embryos transferred in the course of in vitro fertilization and still the proportion of twin births rose. Now new research seems to show that bovine growth hormone in the food supply may be responsible.

Using data obtained from mothers by way of questionnaire, physician Gary Steinman of the Long Island Jewish Medical Center and his colleagues compared the number of twin births from moms who consumed meat and/or milk and those who consumed no animal products at all. They found that the omnivores and vegetarians were five times more likely to have fraternal twins than the vegans.
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Via Organic Consumers Association

Journal of Reproductive Medicine, May 2006: “Mechanisms of Twinning: VII. Effect of Diet and Heredity on the Human Twinning Rate”
G. Steinman

Genotypes favoring elevated IGF and diets including dairy products, especially in areas where growth hormone is given to cattle, appear to enhance the chances of multiple pregnancies due to ovarian stimulation.

Wikipedia on Bovine somatotropin (BGH) –

Bovine somatotropin (bST), or bovine growth hormone (BGH), is a protein hormone that occurs naturally in the pituitary gland of cattle. It is a factor controlling the amount of milk produced by a dairy cow. Bovine somatotropin is naturally in the milk extracted from a cow.
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See also Twin @ Wikipedia