A New York OBGYN specializing in multiple-birth pregnancies has found a startling new reason for the increasing number of twins being born — diet.

By comparing the frequency of twins born to vegans, who consume no animal products, with that of women who eat animal products, Dr. Gary Steinman of Long Island Jewish Medical Center in New Hyde Park, NY, found that women who consume dairy and other animal products are an amazing five times more likely to have twins.

The explanation may be a substance called insulin-like growth factor (IGF), a protein that makes its way into animal milk. IGF increases ovulation and may also help early-stage embryos survive. The concentration of IGF in the blood is about 13 percent lower in vegan women than in women who consume dairy.

In the U.S., twin births has increased significantly since 1975. This has always been attributed to new fertilization techniques and a social trend toward childbearing later in life. "The continuing increase in the twinning rate into the 1990's, however, may also be a consequence of the introduction of growth-hormone treatment of cows to enhance their milk and beef production," said Dr. Steinman.