In a study of more than 50,000 pregnancies in the United States, the odds of having fraternal twins (twins formed from two eggs), increased as maternal body mass index (BMI) rose. “The influence of maternal weight as a factor for twinning will continue to grow in importance as the percentage of obese women in the US continues to rise,” the researchers state.

The increasing use of fertility drugs is thought to be the main reason for a recent rise in twin births in developed countries. To eliminate this potential factor, Dr. Uma M. Reddy, from the National Institute of Child Health and Human Development in Bethesda, Maryland, and colleagues evaluated births that occurred between 1959 and 1966, a period that predated the widespread use of fertility agents.

Increasing BMI was significantly associated with a higher rate of total twin births, the researchers note in the medical journal Obstetrics and Gynecology. However, on further analysis, a statistically significant association was only seen with the birth of fraternal twins, not identical twins (twins formed from the same egg.)

The authors also found that the rate of fraternal twin births rose as maternal height increased. However, the likelihood of fraternal twin births among women in the tallest quartile of height was not as great as that observed in women with a BMI over 30, indicating obesity.

These findings confirm that there is an association between maternal weight and the increased rate of fraternal twin births in the US population, which is not affected by the use of fertility drugs, the team concludes. These results also support findings reported for a Danish population.

SOURCE: Obstetrics and Gynecology, March 2005.