TUESDAY, April 6, 2004 (HealthDayNews) -- Women with a body mass index in the obese range -- 30 or higher -- are more likely to give birth to twins than are normal weight or underweight women.
That's the conclusion of a new Danish study that also found the twins of obese women were slightly more likely to be opposite-sex than same-sex twins.
"We do not know the mechanism," said lead study author Dr. Olga Basso, a research associate professor at the University of Arhus in Denmark. The findings appear in a research letter in the April 7 issue of the Journal of the American Medical Association.
"Given that we do not know the mechanism, I would not be able to speculate on whether losing weight would reduce the risk of twins in these [obese] women," she said. On the other hand, she added, "I would not advise women who want twins to gain weight."
A BMI of 25 or higher is termed overweight, while 30 and above is considered obese. A woman 5-feet, 4-inches tall who weighs 145 pounds has a BMI of 25; if she weighs 175, her BMI is 30.
Basso's team set out to explain why the rates of twin births declined from the 1950s through the 1970s in many countries, but then began to increase again in the 1980s.
The majority of twin births resulted in fraternal, rather than identical, offspring. Fraternal twins are the result of two eggs being fertilized, rather than one egg being fertilized and then separating into two. Factors believed to be behind twin births include maternal age, family history, race and possibly height and weight, Basso said.
Basso isn't the first researcher to report a potential association between maternal weight and twins, but she thinks hers is probably the largest study to date.
She and her team used the Danish National Birth Registry to analyze information on more than 55,000 births, both single and twin, from 1998 to 2001. The overall rate of "twinning" was 2.2 percent, close to the national rate of 2 percent for the same time period.
The researchers also took into account infertility treatments, finding the rates of twins were 15.5 percent among those who had such treatment, and 1.3 percent among those who did not.
They also found that the higher the BMI among untreated women, the greater the likelihood of twin births, with opposite-sex twins slightly more likely than same-sex twins.
For instance, among women not treated for infertility, those with a BMI of 30 or above were nearly 1.5 times more likely to have twins than the women in the normal BMI range of 20 to 24.9. And those with a BMI of 30 or more who were not treated for infertility were 1.6 times were likely to have opposite-sex twins.
The rise in twin births may parallel the rise in obesity, Basso said. In the early 1960s, 9.3 percent of American women ages 20 to 39 had a BMI of 30 or more, but by 2000, 28.4 percent did, she said.
Dr. William Bates, a spokesman for the American Society of Reproductive Medicine, called the new research "an interesting observation." Bates, who is a clinical professor of obstetrics and gynecology at Vanderbilt University, added that hormonal differences between normal weight and overweight women may be at the root of the differences in twin births.
"My speculation is, you have more ovarian follicles primed for ovulation," Bates said. Overweight women have higher levels of leutenizing hormone, which triggers ovulation, he said. Thus, overweight women may release two eggs at once during a monthly cycle.
Bates suggests a study should be done on women in the United States, which has an obesity rate three times that in Denmark.
Both Bates and Basso stopped short of advising women to lose or gain weight to increase or decrease the odds of having twins.
"We don't know if this is universal," Bates said. But there are plenty of other reasons to have a healthy BMI, both Bates and Basso said, including maintaining overall health and avoiding pregnancy complications due to excess weight.