The assumption that the increased use of fertility drugs and assisted reproductive technologies (ART) in the past two decades has partly contributed to the increased risk of fraternal twins is erroneous, says a new study.1 Instead, the research suggests that mothers with a higher body mass index (BMI) face an increased risk, regardless of fertility drug use or the use of ART.
Body mass index is a measure of a person's healthy weight by calculating their weight in relation to their height.2
Obesity May be Tied to Twin Risk
The study by researchers at the National Institutes of Health (NIH) suggests that the odds of having fraternal twins in the United States increase as maternal BMI rises. Specifically, women with a pre-pregnancy BMI of at least 30—considered the threshold of obesity—face this increased risk.
"Increasing use of fertility drugs is considered the primary cause of the recent increase in [fraternal] twinning in most developed countries," wrote Uma Reddy, MD, MPH, with the National Institute of Child Health and Human Development, who led the study. "However, a recent report from a Danish population indicates that twinning is more common among obese than nonobese women."
"Because obesity has become more prevalent among reproductive-aged women over the past several decades, this association is of considerable public health importance," Reddy and her colleagues added.
Thus, the researchers set out to confirm this hypothesis.
Examining Other Possible Causes
To eliminate the potential impact of fertility drugs on the risk of having fraternal twins, Reddy's team evaluated birth records of more than 50,000 women between 1959 and 1966, long before the use of fertility medications became widespread.
After examining the data, Reddy's team found there were 561 twin pregnancies out of the 51,783 records they collected. That translates to 11 twin pregnancies per 1000 pregnancies. Of these, 35% were identical twins, and 46% were fraternal twins.
The research team found that the odds of having fraternal twins increased with a mother's BMI. Those with a BMI of between 25 and 29.9 (considered overweight) faced about one-and-a-half times the risk, whereas those with a BMI of at least 30 (minimum level of obesity), faced more than twice the odds. This was found after taking into account other potential factors that affect the risk, such as maternal race, age, previous pregnancy and maternal height. The risk of having identical twins was not associated with BMI in this study.
The investigators also found that taller women tended to face higher odds of having fraternal twins; however, the effect was not as strong as that of BMI.
In conclusion, the study team wrote: "There has been a marked increase in obesity in the United States, with the proportion of women aged 20 to 39 years with a BMI of 30 or more increasing from 9.3% in the early 1960s to 29.1% in 1999 to 2002."
"This report confirms that obesity is associated with an increased risk of twinning in a diverse U.S. population predating the common use of fertility drugs," they pointed out.
In addition to maternal weight, the increasing age of women at first pregnancy nowadays has also contributed to the higher odds of having fraternal twins, Reddy and her colleagues noted.
Impact of Medications Ruled Out
Increased use of fertility medications, however, is not a factor. "Unlike triplets and other higher order multiples, where 70% are attributable to the use of ovulation-inducing drugs and assisted reproduction, only 18% to 34% of twin births can be attributed to these factors," they wrote.
The reasons behind the link between increased weight and higher odds of having fraternal twins is still unclear, may be due to higher levels of follicle-stimulating hormone (FSH) in these women, the study team theorized. "Increased FSH concentrations have been demonstrated in women with an increased rate of [fraternal] twinning." That still has to be proven, they wrote.
1. Reddy UM, Branum AM, Kelbanoff MA. Relationship of maternal body mass index and height to twinning. Obstet Gynecol 2005 Mar;105(3):593-7.
2. National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK). National Institutes of Health (NIH). Weight-Control Information Network. Understanding Adult Obesity. Available at: http://win.niddk.nih.gov/publications/understanding.htm#bodymass. Accessed March 22, 2005.
John Martin is a long-time health journalist and an editor for Priority Healthcare. His credits include coverage of health news for the website of Fox Television's The Health Network, and articles for the New York Post and other consumer and trade publications.
