The number of U.S. babies born with Down syndrome has increased by nearly one-third over the last three decades, despite the large number of women choosing to end those pregnancies, according to new data from the Centers for Disease Control and Prevention.
CDC researchers say that increase from 1979 to 2003 is probably because more older mothers are giving birth. The prevalence of Down syndrome is five times greater in pregnancies of mothers over age 35.
The number of first births per 1000 women 35 to 39 years of age increased by 36 percent between 1991 and 2001, and the rate among women 40 to 44 years of age rose by 70 percent, according to a 2004 report in the New England Journal of Medicine.
Gov. Sarah Palin had her son Trig last year at the age of 44, an age when the chances of having a child with Down Syndrome is 1 in 35, according to the Eunice Kennedy Shriver National Institute of Child Health and Human Development.
Birthing trends worldwide show that women are waiting longer to have children and advanced maternal age is associated with increased risk of having a child with Down syndrome.
In the December issue of Pediatrics, Mikyong Shin and his colleagues at the CDC reported that the prevalence of Down Syndrome has jumped from 9.0 infants per 10,000 births to 11.8 across 10 geographically, ethnically, and culturally diverse states.
The study is the first broad, population-based effort to look at the prevalence of the syndrome. It conflicts with earlier surveys that projected a decrease in Down syndrome births due to better diagnostic tools and higher abortion rates.
CDC scientists said those numbers are likely to rise over the next several years.
But Dr. Brian Skotko, a pediatric geneticist at Children's Hospital Boston, said the CDC data conflicts with previous data that has demonstrated a decline in the number of Down syndrome births.
Dr. James F.X. Egan and colleagues in the division of maternal-fetal medicine at University of Connecticut Health Center, came to a different conclusions, drawing on data from the National Center of Health Statistics.
They determined that there was a 15 percent decrease in the numbers of babies born with Down syndrome between 1989 and 2005 and that without new testing, there would have been a 34 percent increase in the number of babies born -- an effective 49 percent difference between expected and observed rates.
"The difference likely comes from the data sources that each set of researchers were using," said Skotko. "In either case, the babies born with Down syndrome is far less (49 percent by Egan calculations and 19 percent by CDC calculations) between what could have been and what is actually occurring."
Skotko reviewed research based on studies in Britain and other countries.
The number of Down Syndrome births has stayed steady in Britain.
About 400,000 Americans have Down syndrome, the most common genetic condition in the United States, which produces an array of challenges, including retardation, delayed language and slow motor development.
Down syndrome results when an embryo carries three copies of chromosome 21, instead of the normal two copies. The risk of this defect rises as a mother ages.
In 2002, 263 births were reported in women between 50 and 54 years of age.
An estimated 92 percent of all women who receive a prenatal diagnosis of Down syndrome choose to terminate their pregnancies, according to Skotko's review of research worldwide.