From 
Nature:
Eating milk and other dairy products could increase a woman’s chance of having twins, a US doctor is proposing, based on a study of vegan women. The rate of twin births in the United States rose by more than 75% between 1980 and 2003. Some of this can be explained by the use of fertility treatments, which ups the risk of multiple births. But that can’t explain all of the jump, researchers say. Bearing twins is more risky for both mother and child than having a single baby, so scientists want to know what’s causing the rise.
Gary Steinman of the Long Island Jewish Medical Center in New Hyde Park, New York, carried out a simple comparison: he gathered together childbearing records for more than 1,000 vegan women who do not eat any animal products. He calculated that vegans were around five times less likely to bear twins than omnivorous women or vegetarians who eat dairy food.
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