Skip to content

    Health & Pregnancy

    Font Size

    Moms' Obesity Makes Twins More Likely

    Maternal Obesity Increasingly Important Factor for Fraternal Twins
    WebMD Health News

    March 3, 2005 -- 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.

    What About Triplets?

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

    The trend between mothers' prepregnancy 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.

    Pregnancy Week-By-Week Newsletter

    Delivered right to your inbox, get pictures and facts on
    what to expect each week of your pregnancy.

    Today on WebMD

    hand circling date on calendar
    Track your most fertile days.
    woman looking at ultrasound
    Week-by-week pregnancy guide.
    Pretty pregnant woman timing contaction pains
    The signs to watch out for.
    pregnant woman in hospital
    Are there ways to do it naturally?
    slideshow fetal development
    pregnancy first trimester warning signs
    What Causes Bipolar
    Woman trying on dress in store
    pregnant woman
    Woman looking at pregnancy test
    calendar and baby buggy
    dark chocolate squares