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A handful of biotech companies are racing to market a new generation of tests for Down syndrome, a development that promises a safer way to spot the most common genetic cause of mental retardation early in pregnancy even as it weaves a thicket of moral, medical, political and regulatory concerns.

Doctors recommend that all pregnant women be offered screening for Down syndrome, and about half of women undergo the tests. But the current tests often produce confusing, ambiguous results, unnecessarily alarming couples or falsely reassuring them. The new tests are designed to offer more definitive results early in the pregnancy.

But with the first new approach due to become available this spring, the tests are renewing questions about why regulators do not require such innovations to be proved reliable before being offered to the public.

Abortion opponents, meanwhile, fear that the technology may prompt more couples to terminate pregnancies. And advocates for the disabled, noting that couples are often poorly informed about the syndrome, worry that more of them may feel pressured to abort. They also fear a dwindling number...