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Scientists are making strides toward designing noninvasive tests that can tell whether a fetus suffers from inherited disorders such as cystic fibrosis and sickle cell anemia that are linked to a single mutated gene.

Prenatal tests for some of these single-gene maladies -- known as monogenic diseases -- are available today, but they require a needle to be inserted into the mother's womb and therefore carry a small risk of miscarriage. A noninvasive version would simply be based on a blood sample taken from the mother.

In a small study, researchers were able to test for genetic mutations in the fetus's free-floating DNA isolated from the mother's blood. "We've shown proof of principle," said Dennis Lo of the Chinese University of Hong Kong and co-author of the study, which appears in the online version of the Proceedings of the National Academy of Sciences.

Dr. Lo acknowledged, however, that the noninvasive process as it now stands was expensive and inefficient, and any clinical test was at least three years away. The latest results would also have to be replicated in a...