Moms' Obesity Makes Twins More Likely
Maternal Obesity Increasingly Important Factor for Fraternal Twins
March 3, 2005 -- Obesity may raise a woman's chances of having fraternal
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
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
Baby Boom for Twins
The mythical baby-carrying stork has pulled double duty a lot over the last
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
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
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.
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
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% are
attributable to the use of ovulation-inducing drugs and assisted reproduction,
only 18%-34% 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
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.