How to Have Twins: Increased Maternal BMI and Twinning

Study shows that overweight mothers more likely to have twins

Maternal BMI and Twinning
Are bigger, taller women more likely to have twins?. Alex Brosa / E+ / Getty Images

As the number of twin births has risen since the middle of the last century, researchers have sought explanations for the surging number of twins. Why are more women having twins? One commonly accepted reason is the rise in usage of fertility drugs and treatments. But another factor is that more women are delaying childbearing until they are older. In addition, researchers have noted another trend that correlates with the rise in twinning, one that may offer further explanation.

Are heavier, taller women more likely to have twins?

Maternal BMI and Twinning

According to a study published in the Journal of Obstetrics and Gynecology, overweight and tall mothers are indeed more likely to have twins. The 2005 study shows that mothers with a BMI index of 30 or more, classified as obese, are significantly more likely to have twins. BMI or Body Mass Index is a measure of body fat based on a height and weight ratio.

Researchers analyzed over 50,000 pregnancies, (561 twin) in the Collaborative Perinatal Project, which took place at several hospitals in the United States. They compared the incidence of twinning with the mothers' prepregnant body mass index (BMI). They concluded that there is an association between women with a higher BMI and twinning. The study notes a "statistically significant trend for increased risk of total twinning with increasing BMI."

The study showed that taller-than-average women also have an increased chance of having twins.

However, the correlation between height and weight is only associated with fraternal or dizygotic twinning. The mother's stature did not impact her chances of having identical or monozygotic twins, according to the study. Fraternal or dizygotic twinning is influenced by a number of different factors, including the use of fertility drugs, heredity, and maternal age.

The causes of monozygotic twinning are not thoroughly understood.

Does that mean that women who are tall or heavy will have twins? Not necessarily. There are plenty of moms of twins who are not overweight and/or short. Likewise, there are plenty of larger moms who have singletons. Rather, the research identified that maternal BMI is a factor that increases the chances of twins, one of many causes of twins. As obesity rates have trended upwards, so have the number of twins.


Fryar CD, Carroll MD and Ogden, CL. "Prevalence of Overweight, Obesity, and Extreme Obesity Among Adults: United States, Trends 1960-1962 Through 2009-2010." National Center for Health Statistics, September 2012.

Reddy, Uma M., Branum, Amy, Klebanoff, Mark A. “Relationship of Maternal Body Mass Index and Height to Twinning.” Obstetrics and Gynecology., March 2005, pg. 593.

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