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Weighing the Odds

March 3, 2005 -- There's yet another reason to make sure you're in good physical health before having a baby. The odds of having fraternal twins in the United States increases with maternal obesity, according to a study in the March issue of Obstetrics & Gynecology.

Researchers looked at birth records of 51,783 pregnancies for the years 1959-1966, which pre-dated the common use of fertility drugs. They found that the odds of fraternal twins (each formed from a separate egg and sperm) were significantly elevated among women with a Body Mass Index (BMI) of 30 or greater.

Here's a look at the breakdown:

BMI Total Pregnancies Twin Pregnancies
< 20 21,923 46 (0.4%)
20-24.9 27,091 122 (0.5%)
25-29.9 8,047 56 (0.7%)
30 and >30 3,421 36 (1.1%)

BMI calculates your weight in kilograms divided by the square of your height in meters. Because it factors in your body fat percentage, BMI is an accurate measurement of your overall fitness, and is considered a more reliable measurement than simply your height and weight alone. A BMI of 18.5 to 24.9 is considered normal, 25 to 29.9, overweight, and 30 to 34.9, obese.

Beyond the obesity link, the study also found a relationship between maternal height and increased odds of fraternal twins. Women in the top 25th percentile of height had significantly increased odds ratio for fraternal-twin pregnancies, although this association was not as strong as that for BMI.

Study authors note that the proportion of U.S. women ages 20-39 with a BMI of 30 or more increased from 8.3 percent in the early 1960s to 19.1 percent in 1999-2002. Thus, the authors say, the rate of twin births is likely to increase as the percentage of obese women in the U.S. continues to rise.

The U.S. incidence of fraternal twins increased by 65 percent between 1980 and 2002, from 18.9 to 31.1 per 1,000 live births. An estimated one-third of this increase in multiple births has been attributed to the trend of increased maternal age at birth; the remainder was presumed to be the result of increased use of fertility drugs and assisted reproductive technologies since the early 1980s.

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