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Paving the Way to Prevention
New Study Shows Increased Prevalence of Cardiovascular Birth Defects in Twins

"Regardless of what is causing the increase in twin births, it's important for us to understand what is going on in terms of birth defects in this increasing population of babies."

In a new study published in the American Journal of Medical Genetics, scientists in the March of Dimes Research Division at CHORI have shown that twins had a higher rate of cardiovascular birth defects when compared to singletons in 16 different categories of heart defects.

“We compared twins with cardiovascular defects to singletons with the same defects and determined that twins have a much higher prevalence,” explains Jill Harden, PhD. “We also stratified twins by like sex and unlike sex and determined that like sex twins had an increased prevalence in seven of sixteen categories.”

In general, cardiovascular birth defects occur in about 1 out of every 100 live births, or about one percent of babies born in the United States. They range in severity, with some heart defects being very life-threatening, while others can be treated more easily. In many cases, surgery can repair the defect and the child can live a normal life.

While the prevalence of cardiovascular birth defects have been reasonably well described in children born singly, there has been much less research conducted on twins, even though the number of twin births has been steadily increasing.

"In California, the twin population has been getting steadily larger since about 1992. We don't know why, though there is of course a suspicion that the increase is due to increased use of fertility enhancements," says Dr. Hardin. "Regardless of the cause, it's important for us to understand what is going on in terms of birth defects in this increasing population of babies."

Prior studies on twins have been hampered by small samples, but because CHORI and the March of Dimes California Research Division partnered with the State of California Birth Defects Monitoring Program, Dr. Hardin and her colleagues were able to uniquely examine a significantly larger sample population.

"This is a much larger study than has ever been done in this realm before. Unlike any other study, we were actually able to investigate the prevalence of specific heart defects rather than just broad classes of defects," explains Dr. Hardin.
"In addition, California is made up of a pretty diverse population, so the fact that we have different racial and ethnic groups represented also makes this study very unique."
With over 50 thousand twin pair samples, the new study provides definitive evidence of the overall increased prevalences in twins in all 16 categories of cardiovascular birth defects, with twins presenting with at least double the prevalence in 7 of the 16 categories. In addition, however, Dr. Hardin and her colleagues also determined that like-sex twins had an increased prevalence of cardiovascular birth defects as compared with unlike-sex twins.

"Same-sex twins suggest identical twins, with identical genetic make-up," Dr. Hardin says. "The fact that more same-sex twins than different-sex twins were likely to have certain defects suggests that there is more of a genetic underpinning to those defects than other defects."

This finding was underscored by subsequent literature review, which showed that these specific types of cardiovascular defects were more likely to be recurrent within families, again pointing to a genetic link.
The results from the current study pave the way for additional research into the causes of the increased prevalence of heart defects in twins, such as molecular genetics investigations to identify certain genetic regions that might hold the key to some of these cardiovascular birth defects.

As Dr. Hardin says, "This study provides an excellent initial step toward those kinds of genetic studies that would identify which genes might be relevant in figuring out what the difference is between twins and singletons, and why these birth defects are occurring in the first place."

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Tuesday, May 17, 2011 8:19 AM

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