Gary Steinman, an obstetrician at New York's Long Island Jewish Medical Center, compared the heights of 129 women who gave birth to twins or triplets with the average height of women in the United States and found women with multiple-birth pregnancies averaged more than an inch taller.

Steinman, whose study is being published in the September issue of the Journal of Reproductive Medicine, said the reason seems to be an insulin-like growth factor, which has been positively linked to both height and twinning.

Insulin-like growth factor is a protein that is released from the liver in response to growth hormone. It increases the sensitivity of the ovaries to follicle stimulating hormone, thereby increasing ovulation, the study said.

"Any circumstance that affects the amount of available insulin-like growth factor so as to modify the sensitivity of the ovary to follicle-stimulating hormone appears to govern the rate of spontaneous twinning," Steinman said in a statement.

Previous studies have demonstrated that shorter people have significantly lower levels of this protein and that countries with taller women have higher rates of twinning compared to countries with shorter women.

In a previous study, Steinman found that women who consume animal products, specifically dairy, were five times more likely to have twins. Cows, like humans, produce insulin-like growth factor in response to growth hormones and release it into the blood, and the protein makes its way into their milk.