Pregnant women seeking prenatal tests for genetic defects face difficult choices. Either they accept less-than-reliable blood tests or ultrasound interpretations that leave them anxious and guessing, or they choose amniocentesis, which punctures the embryonic sac and has a small risk of causing miscarriage.

Researchers have long envisioned the day when a test of the mother's blood or urine could conclusively detect a genetic defect in her baby. Xenomics, a New York City-based biotech firm, is now conducting the first clinical studies of a urine test that, it says, can detect Down syndrome in the fetus.