updated 1/5/2006 3:27:10 PM ET 2006-01-05T20:27:10

Down syndrome in the United States is more common than previously thought, at one case for every 733 live births, according to a new government report containing what are regarded as the most reliable estimates yet on the prevalence of 18 types of birth defects.

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Previously, Down syndrome, a type of retardation caused by a genetic mutation, was estimated to occur in a range of one in every 800 live births to one in every 1,000.

The report, released Thursday by the Centers for Disease Control and Prevention, also found that cleft lip occurs in about one in every 1,000 births, and cleft palate in about one in every 1,500.

The report was celebrated by advocacy groups that help families affected by birth defects. They noted that the new numbers are based on statewide data, while previous estimates were derived from selected clinics and hospitals.

“Until now, there’s been a real dearth of good, reliable, national statistics on Down syndrome,” said Suzanne Armstrong, spokeswoman for the National Down Syndrome Society.

The risk of Down syndrome increases with the age of the mother, from an estimated 1 in 2,000 among 20-year-old women to 1 in 100 for women age 40. Many women are having babies later in life, which might explain the higher rate in the new study.

But because the new statistics were not collected in the same way as the old, it is not clear whether Down syndrome has really increased, Armstrong and others said.

From 1974 until the mid-1990s, the government tracked birth defects through a system that relied on hospital discharge reports. But not all hospitals were represented, and researchers said it was possible that the participating hospitals attracted more or fewer problematic births than the norm.

In 1997, the government collected data from all birth hospitals, clinics and referral centers in certain parts of the country.

The new report presents statewide data for Arkansas, Hawaii, Iowa, Massachusetts, North Carolina, Oklahoma, Texas and Utah. It also has data from the metropolitan Atlanta area, the Central Valley in California and a group southern Alabama counties. The statistics are for the years 1999 to 2001.

The 11 states account for about 22 percent of all live births in the United States each year and are a good representative sample, said Joann Petrini, a study co-author and director of the March of Dimes’ perinatal data center in White Plains, N.Y.

The researchers included only 18 of the thousands of birth defects that have been identified by doctors and scientists. Missing are common birth defects such as clubfoot, hip dislocation and hypospadias, in which a boy’s urinary opening is on the underside of the penis instead of the tip. Spina bifida was not included either, because health officials already have what they consider reliable estimates.

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