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"Our results suggest that the risks of specific types of birth defects in twins may be different depending on whether fertility treatments were used," said study lead author April Dawson, a health scientist with the National Center on Birth Defects and Developmental Disabilities at the U.S. Centers for Disease Control and Prevention.
Jeffrey Roth, a research professor of pediatrics with the University of Florida, said the findings "may begin to lower the anxiety of women who receive fertility treatment that their offspring face an elevated risk of birth defects."
Still, he said the findings are incomplete due to limited statistics about babies and their birth defects, meaning that "the take-home message from this study needs to be considered very cautiously."
Dawson didn't disagree. "The number of women in our study who reported use of fertility treatments was very small, limiting our ability to draw conclusions about the risk of birth defects among twins in this group."
According to U.S. statistics, twin births have jumped 75 percent since 1980, accounting for 3.5 percent of births in 2014.
Two factors explain the rise in twins -- the growth in the use of fertility treatments and greater numbers of women having kids later in life. Both boost the twin rate, specialists say.
The higher twin rate hasn't affected the birth defect rate, however. About 3 percent of babies born each year in the United States have birth defects, or one in 33, said Dawson.
"Although the birth rate of twins has increased over time, twins are still a relatively small proportion of live births, so the impact of twin births on the overall rate of birth defects is small," she noted.
Dawson and her colleagues launched the new study to better understand the risks of various birth defects in twins and gain insight into the role of fertility treatments on these abnormalities.
For the study, they analyzed a database of babies born from 1997-2007 in 10 U.S. states. They found the odds of several types of birth defects were higher in twins.
Those who underwent fertility treatments were linked to higher risks of five of 25 defects, but the risks for several couldn't be calculated due to small numbers of mothers who used fertility treatments. Conceiving twins naturally was linked to higher risks of 29 of 45 defects, although again, some risks couldn't be calculated.
"Among mothers who did not use fertility treatments to get pregnant, twins were at increased risk of many different types of birth defects, in particular ventricular septal defects [defects of the wall separating the left and right sides of the heart] and cloacal exstrophy [a defect of the abdominal wall]," Dawson said.
"Among mothers who used fertility treatments to get pregnant, twins were at increased risk of several different defects of the heart, omphalocele [a defect of the abdominal wall] and a defect of the eye," she added.
It's not clear why these differences in risk exist, Dawson said. As for the overall higher risk of birth defects in twins, she said there are several possibilities. Something may cause both twins and birth defects, she theorized. Or twin fetuses may suffer from a lack of space in the uterus, cutting down on blood supply and proper nutrition.
Roth stressed that the study is limited because it only goes to 2007, although the rate of twins has continued to rise since then. More recent statistics might reveal different connections between twins, fertility treatments and birth defects, he said.
The study was published June 20 in the Journal of Epidemiology & Community Health.
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