The study, published online today in the American Academy of Pediatrics' journal Pediatrics , showed one in 730 babies born in Utah had the genetic disorder, compared with one in 848 among the 10 states studied.
The study didn't explore why. But a Utah expert on birth defects says it's likely because of Utah mothers' ages and attitudes.
Many continue to have children into their late 30s and 40s, which increases the risk of the chromosomal disorder, said Lorenzo Botto, a medical epidemiologist at the Utah Birth Defect Network, which provided data for the study.
And Utah women are less likely to have abortions once the condition has been detected prenatally, he added.
The higher prevalence is "not because there is something wrong with Utah," he said in an e-mail, "but is basically a function of family choices."
Amy Moore, whose 14-year-old daughter has Down syndrome, was not surprised by Utah's ranking. When she lived here -- she now lives in Wyoming -- Kenly had friends with the condition and Moore had a larger support system.
Moore said her teenager is a cheerleader, swims and after years of speech therapy can "say whatever is on her mind, for better or for worse. She's nothing but a blessing in our lives."