Science Blog has a press release regarding the research of Gary Steinman on dietary influences on twinning. I'm going to cite a lenghty passage, because it's pretty interesting, and it's a press release:
[T]he women who consume animal products, specifically dairy, are five times more likely to have twins.
The culprit may be insulin-like growth factor (IGF), a protein that is released from the liver of animals -- including humans -- in response to growth hormone, circulates in the blood and makes its way into the animal's milk. IGF increases the sensitivity of the ovaries to follicle stimulating hormone, thereby increasing ovulation. Some studies also suggest that IGF may help embryos survive in the early stages of development. The concentration of IGF in the blood is about 13 percent lower in vegan women than in women who consume dairy.
The twinning rate in the United States has increased significantly since 1975, about the time assisted reproductive technologies (ART) were introduced. The intentional delay of childbearing has also contributed to the increase of multiple-birth pregnancies, since older women are more likely to have twins even without ART.
"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.
In the current study, when Dr. Steinman compared the twinning rates of women who ate a regular diet, vegetarian diet with dairy, and vegan diet, he found that the vegan women had twins at only one-fifth the rate of women who commonly do not exclude milk from their diets.
In addition to a dietary influence on IGF levels, there is a genetic link in numerous species of animals, including humans. In cattle, regions of the genetic code that control the rate of twinning have been detected in close proximity to the IGF gene. Researchers have found through large population studies of African American, Caucasian and Asian women that blood IGF levels are greatest among African Americans and lowest in Asians. Some women are just genetically programmed to make more IGF than others. Twinning rates in these demographic groups parallel the IGF levels.
"This study shows for the first time that the chance of having twins is affected by both heredity and environment, or in other words, by both nature and nurture," said Dr. Steinman. These findings are similar to those observed in cows by other researchers, namely that a woman's chance of having twins appears to correlate directly with her blood level of insulin-like growth factor.
"Similar to those observed in cows" is NOT the way you want to refer to twin mothers, by the way. I know one twin mother who would be markedly less likely to participate in that research.
Of course, they're monozygotic, so dairy is irrelevant.
In any event, I find it quite incredible that vegan women have twins at only one-fifth the rate of non-vegans. That seems like more of a difference than 13 percent IGF differences would explain, unless there was a large variance in levels and the effect was concentrated in the upper tail. Maybe there are other dietary factors also.
As far as growth hormone dairy is involved, it ought to be possible to find that out from women who consume organic or BGH-free milk.
And now for some twin research trivia:
Previous twinning studies
Dr. Steinman found that women who become pregnant while breastfeeding are nine times more likely to conceive twins than women who are not breastfeeding at the time of conception. He also confirmed findings by others that identical twin sets are more often female than male, especially in conjoined twin sets, and that monozygotic twin sets are more likely to miscarry than dizygotic sets. Dr. Steinman also found evidence through fingerprint analysis that as the number of fetuses in a monozygotic set increases, so does the level of physical diversity among them. In his most recent study of the mechanisms of twinning prior to the new study, Dr. Steinman confirmed that use of in vitro fertilization (IVF) methods increases the incidence of monozygotic twinning -- where the transfer and/or implantation of two embryos results in three infants -- and he proposed that adding more calcium or reducing the chelating agent ethylenediamine tetraacetic acid (EDTA) in the IVF incubation media might decrease the unwanted complication.
I suppose that observation about physical diversity would predict that clones carried apart ought to be more similar in appearance than identical twins carried together.