Taller women are more likely to have twins, according to a US study.
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.
美国一项研究表明，个子高的女性生双胞胎的可能性较大。
纽约长岛犹太医疗中心的妇产科医师加里·斯汀曼将129名生过双胞胎或三胞胎的女性的身高与美国女性的平均身高进行了对比，结果发现生过多胞胎的女性平均身高要高出2.54厘米。
斯汀曼的此项研究结果在《生殖医学》的第九期上发表，他说，这可能和类胰岛素生长因子有关，类胰岛素生长因子与身高和孪生几率呈正相关关系。
类胰岛素生长因子是人体肝脏在生长激素的作用下分泌出的一种蛋白质。研究表明，类胰岛素生长因子能增强卵巢对促卵泡激素的敏感度，从而促进排卵。
斯汀曼在一份声明中提到："任何能影响类胰岛素生长因子分泌量的因素都会改变卵巢对促卵泡激素的敏感度，从而影响自然孪生的几率。"
之前已有研究表明，个矮的人体内分泌的类胰岛素生长因子量要低很多，此外，女性个子较高的国家比女性个子较矮的国家的双胞胎出生率高。
斯汀曼在此前的一个研究中发现，常食用畜产品尤其是奶制品的女性生双胞胎的几率比不食用这些食品的女性大五倍。奶牛和人一样，也能在生长激素的作用下分泌类胰岛素生长因子，这种蛋白质被释放到血液里，从而进入到牛奶中。
Vocabulary:
insulin-like growth factor :类胰岛素生长因子（IGF）