The rate of twin births has also increased significantly since 1975, when assisted reproductive technology came into wide use. But these factors alone, Dr. Steinman said, do not explain the continuing increase in the rates in the United States since 1994, when recombinant bovine somatotropin was approved for sale.
In 2003, the United States had 3 sets of twins per 100 live births — more than twice the rate of Britain, where growth hormone injection is banned. (Triplets and higher multiple births raise this figure to 3.18.)
Rise in Rate of Twin Births May Be Tied to Dairy Case
By NICHOLAS BAKALAR
American women who eat dairy products appear to be five times as likely to give birth to fraternal twins as those who do not, according to a new study, and one explanation may lie in dairy products from cows injected with synthetic growth hormone.
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.
Multiple births are associated with increased health risks for mothers and infants, but Dr. Steinman said he was not prepared to use these findings as the basis for advising women about diet before pregnancy.
"Since this is the first time diet has been implicated in an important role for determining twinning rate," Dr. Steinman said in an e-mail message, "it must be confirmed by others before rigid recommendations can be made concerning health care."
