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New prenatal blood tests for genetic abnormalities such as Down syndrome are reshaping care for expectant mothers, but their rapid rollout has raised fears that poorly understood results could lead to confusion among patients and doctors managing high-risk pregnancies.

Four companies, including Sequenom Inc. and a unit of gene-sequencing firm Illumina Inc., are vying for the new market for the tests, which examine traces of fetal DNA in the mother's bloodstream. The market has grown rapidly since the tests were first introduced in late 2011-with Sequenom alone topping 61,000 tests in 2012. The industry expects that at current rates it will run hundreds of thousands of tests this year, and many doctors believe they could fast become the standard of care for pregnant women.

At the same time, case reports of inaccurate results-though rare-are turning up more frequently than expected, medical specialists say. They worry that in the worst-case scenario, inaccurate test results could contribute to the abortion of healthy babies.

Members of the American College of Medical Genetics and Genomics question whether the products should be called "tests" at...