Obesity (search) may raise a woman’s chances of having fraternal twins (search).

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 percent in the early 1960s to 29 percent 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 percent 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 percent were identical twins, 46 percent were fraternal twins, and 19 percent weren’t identified one way or the other.

The mothers also disclosed their pre-pregnancy height and weight. Using those numbers, the researchers calculated the women’s body mass index (search) (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.

What About Triplets?

The researchers found that the odds of identical-twin pregnancy were not related to increased pre-pregnancy weight, but the odds of a fraternal-twin pregnancy were increased in women with a BMI of 30 or greater.

The trend between mothers’ pre-pregnancy weight and fraternal twins has also been noted in other countries. Studies from the U.K. (specifically, Scotland), France, Nigeria and Denmark have shown the same pattern.

The study’s tallest women were also significantly more likely to have twins. However, the link wasn’t as strong as the one between twin births and increased BMI.

Fertility drugs and reproductive technology account for most other multiple births, including triplets, say the researchers.

“Unlike triplets and other higher-order multiples, where 70 percent are attributable to the use of ovulation-inducing drugs and assisted reproduction, only 18-34 percent of twin births can be attributed to these factors,” write Uma Reddy, MD, MPH, and colleagues.

Reddy works at the pregnancy and perinatology branch of the National Institute of Child Health and Human Development, which is part of the National Institutes of Health (NIH). The study appears in the March 2005 issue of Obstetrics & Gynecology.

The researchers conclude that their work confirms an association between maternal weight and fraternal twins independent of the use of fertility drugs.

Twins are at risk for a variety of adverse pregnancy and delivery outcomes and have higher death and disability rates compared with single-birth outcomes. The influence of maternal weight on fraternal twins will continue to grow in importance as the percentage of obese women in the U.S. continues to rise.

By Miranda Hitti, reviewed by Brunilda Nazario, MD

SOURCES: Reddy, U. Obstetrics & Gynecology, March 2005; vol 105: pp 593-597. News release, American College of Obstetrics and Gynecology.