A new test was introduced Monday that can determine if a fetus has Down syndrome using a sample of the mother’s blood. The test, and others like it, are expected to reach the market in the coming year and might eventually reduce the need for invasive tests that carry a slight risk of inducing miscarriages.

But some advocates for those with Down syndrome fear the new tests, which can be conducted as early as the 10th week of pregnancy, will lead to more abortions and reduce the population of those with Down syndrome. And they lament what they say is the perception that lives with Down syndrome are not worth living.

Dr. Brian G. Skotko, a specialist in the Down syndrome program at Children’s Hospital Boston, said that the number of babies born annually with Down syndrome in the United States declined 11 percent from 1989 to 2006. This was during a period when the number of such births would have been expected to increase by 42 percent because more women were putting off child-bearing until they were older, when the risk of an affected pregnancy increased.

The reason is that most women who find they are carrying a fetus with Down syndrome, which causes mild to moderate mental retardation, terminate the pregnancy.

Yet most women deemed at a higher-than-usual risk of an affected pregnancy do not get the invasive tests – amniocentesis or chorionic villus sampling – that can diagnose Down syndrome in the fetus.

Marcy Graham, a spokeswoman for Sequenom, the company that introduced the new test Monday, said there were an estimated 750,000 high-risk pregnancies a year in the United States, but only 200,000 invasive tests.

One reason women forgo testing is that many are willing to have a baby with Down syndrome. But there are others who avoid the invasive tests because they have a slight risk – often cited as one in 200 but probably lower – of inducing a miscarriage.

“This is an absolutely every-day occurrence for me that I talk to someone who is 37 years old and doesn’t want a Down syndrome baby but doesn’t want to go through an invasive procedure,’’ said Dr. Stephen A. Brown, associate professor of obstetrics and gynecology at the University of Vermont.

The new tests should eliminate that miscarriage risk, leading to a big upsurge in testing and Down syndrome diagnoses, and possibly more abortions.

“Will we slowly start to see babies born with Down syndrome disappear?’’ asked Dr. Skotko, who has a sister with the condition.

He and some colleagues recently published the results of a survey in which nearly 99 percent of people with Down syndrome said they were happy with their lives.

Parents of such children also said they were happy. About 79 percent of parents said their outlook on life was more positive because of their child.

There is also an upsurge in efforts to develop drugs to improve the learning ability of those with Down syndrome. One of the drug researchers, Alberto Costa, who has a daughter with the condition, told The New York Times Magazine:

“It’s like we’re in a race against the people who are promoting those early screening methods. These tests are going to be quite accessible. At that point, one would expect a precipitous drop in the rate of birth of children with Down syndrome. If we’re not quick enough to offer alternatives, this field might collapse.’’

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