Washington, DC -- A study in the March issue of Obstetrics & Gynecology finds that the odds of having fraternal twins in the US increase with maternal obesity. Women with a pre-pregnancy body mass index (BMI) of 30 or more are significantly more likely to have dizygotic twins (fraternal twins, each formed from a separate egg and sperm) than are women with lower BMIs. The study also found a relationship between maternal height and increased odds of fraternal twins. Women in the top 25th percentile of height had a significantly increased odds ratio for fraternal-twin pregnancies, although this association was not as strong as that for BMI.

The US incidence of fraternal twins increased by 65% 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. The chance of having identical twins (twins that come from one egg) is usually a random event with similar rates worldwide of less than one occurrence every 100 births.

Researchers from the National Institute of Child Health and Human Development and the Centers for Disease Control and Prevention looked at birth records of 51,783 pregnancies for the years 1959-1966, which predated the common use of fertility drugs. They found that the odds of identical-twin pregnancy were not significantly related to increased pre-pregnancy weight, but the odds of fraternal twins were significantly elevated among women with a BMI of 30 or greater.

Study authors note that the proportion of US women ages 20-39 with a BMI of 30 or more increased from 9.3% in the early 1960s to 29.1% in 1999-2002. Thus, the rate of twin births is likely to increase as the percentage of obese women in the US continues to rise.

Contact: Uma M. Reddy, MD, MPH, at the Pregnancy and Perinatology Branch, National Institute of Child Health and Human Development, Bethesda, MD, at reddyu@mail.nih.gov.

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