Obesity may raise a woman's chances of having fraternal twins.

Women who were obese before pregnancy are significantly more likely to give birth to fraternal twins, a new study shows. However, identical twins were not linked to maternal obesity. Fraternal twins are born from separately fertilized eggs.

America's obesity trend could partly explain why twins have become much more common, the researchers write. The increase in the obesity epidemic continues in the U.S., say the authors. The proportion of women of childbearing age with a BMI of 30 or more increased from 9% in the early 1960s to 29% in 1999-2002.

Baby Boom for Twins

The mythical baby-carrying stork has pulled double duty a lot over the last 25 years.

America's fraternal twin birth rate soared 65% between 1980 and 2002, the study shows. That's an increase from 19 to 31 out of every 1,000 live births.

Before now, the rise in twin births was chalked up to increased maternal age, fertility drugs, and assisted reproduction technology.

But those reasons don't explain the new study's findings.

The study covered more than 51,000 live births across the U.S. from 1959 to 1966. Back then, fertility drugs and other reproductive technologies weren't common.

Twins accounted for a total of 561 pregnancies. That's 11 out of every thousand babies. Of those, 35% were identical twins, 46% were fraternal twins, and 19% weren't identified one way or the other.

The mothers also disclosed their prepregnancy height and weight. Using those numbers, the researchers calculated the women's body mass index (BMI). A BMI of 30 or higher is obese.

Increased BMI was significantly related to the odds of having fraternal twins, says the study. Maternal age didn't change that.