NEW HYDE PARK, NY -- An obstetrician who specializes in multiple-birth pregnancies has confirmed that taller women are more likely to have twins. The suspected culprit is insulin-like growth factor, which has been positively linked to both height and twinning. By comparing the heights of women who had given birth to twins or triplets with the average height of women in the United States, Gary Steinman, MD, PhD, an attending physician at Long Island Jewish (LIJ) Medical Center, found that the multiple-birth mothers averaged more than an inch taller. The study was published in the September issue of the Journal of Reproductive Medicine.
"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," said Dr. Steinman.
Insulin-like growth factor (IGF) 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. Some studies also suggest that IGF may help embryos survive in the early stages of development.
Among its many effects in the body, IGF stimulates cells in the shaft of long bones to grow. Previous studies have demonstrated that people with short stature have significantly lower levels of IGF. Countries with taller women have higher rates of twinning compared to countries with shorter women.
In the current study, Dr. Steinman compared the heights of 129 women who gave birth to identical or fraternal twins or triplets -- 105 had twins and 24 had triplets -- with the average height of women in the United States, as reported by the National Center for Health Statistics. The multiple-birth mothers averaged 5 feet 5 inches tall, more than an inch taller than the U.S. average for adult females of about 5 feet 3 ¾ inches. While the effect of IGF on the ovaries likely involves fraternal, or dizygotic, twins, they were not distinguished from identical, or monozygotic, pregnancies in this study. Dizygotic twin pregnancies account for about two-thirds to three-quarters of all spontaneous multiple pregnancies in a random population, therefore the results of this study predominantly, but not exclusively, represent fraternal twins.
In the previous study in his series on the mechanisms of twinning, Dr. Steinman found that women who consume animal products, specifically dairy, are five times more likely to have twins. Cows, like humans, produce IGF in response to growth hormone and release it into the blood, and the IGF makes its way into their milk.
Dr. Steinman has been invited to speak next month about IGF and twinning at the three-day workshop "Milk, Hormones and Human Health." The meeting, to be held in Boston from Oct. 23-25, is sponsored by the Harvard Center for Cancer Prevention at the Harvard School of Public Health and the McGill University Centre for Cancer Prevention.