A multiple-birth pregnancies specialized obstetrician affirmed that taller women are more likely to have twins. Taller women have greater levels of an insulin-like growth factor (IGF) that has been linked to height and to the incidence of twins in previous studies.

Comparing the heights of twins or triplets mothers with the average height of women in the US, Gary Steinman, an attending physician at Long Island Jewish (LIJ) Medical Center, found that the multiple-birth mothers averaged 5 feet 5 inches (1.65 m ) tall, more than an inch (3.2 cm) taller than US average women 5 feet 3 ¾ inches .

"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.

The liver releases IGF as a feed back response to growth hormone. It produces multiple ovulation by rising the receptivity of the ovaries to follicle stimulating hormone. Some also suggest that IGF may help embryos survive in the early stages of development. IGF stimulates cells in the shaft of long bones to grow.

It is 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 this study dizygotic twins (which derive from different ovules) were not distinguished from identical (monozygotic) twins which derive from one ovule. IGF likely involves dizygotic twins.

Dizygotic twin pregnancies account for about 66 to 75 % of all multiple pregnancies in a population, therefore the results of this study predominantly, but not exclusively, represent fraternal twins.

Other studies discovered that women who consume animal products, specifically dairy, are five times more likely to have twins. Cows also produce IGF and from their blood it passes into their milk.