People differ in the size and composition of their chromosomes, which can affect the test results

IMAGE:This is conceptual art of maternal blood screening of fetal DNA. view more

Credit: Alice C Gray

Prenatal blood screening for extra or missing chromosomes in the fetus might give false-positive results if the mother's genome contains more than the usual number of certain DNA segments. This finding is reported April 1 in the New England Journal of Medicine. The article is part of a collection of papers examining screening tests now available to patients due to recent advances in genome sciences.

Researchers at the University of Washington, Fred Hutchinson Cancer Research Institute, and the Howard Hughes Medical Institute worked together to analyze false-positive results from the newer prenatal genetic screens.

Dr. Hilary Gammill, UW assistant professor of obstetrics and gynecology and research associate at Fred Hutchinson Cancer Research Institute, and Dr. Jay Shendure, UW professor of genome sciences, are the senior authors of the study. The lead authors are Matthew W. Snyder, UW genome sciences graduate student, and Dr. Lavone Simmons, former UW fellow in maternal-fetal medicine.

The newer prenatal genetic screens analyze cell-free DNA circulating in the mother's blood during pregnancy. The tests are safer and less invasive than sampling the fluid surrounding the fetus in the uterus.

The blood tests are now routinely offered to pregnant women whose offspring might face greater odds of certain genetic conditions, such as the chromosome trisomies that are more common in children born to older mothers. In a trisomy, there three, instead of the usual two, copies of a particular chromosome. Some trisomies, such as Edwards and Patau syndromes, cause life-threatening medical problems and have high stillbirth and newborn mortality rates.

Based on previous investigations, the new screening tests reportedly have a high accuracy in pregnancies that are at high risk for aneuploidy (extra or missing chromosomes), as well as in pregnancies that are at low risk.

The overall reduced incidence of uneven chromosome counts in low-risk pregnancies, however, limits the positive predictive value of these non-invasive prenatal screening tests. Researchers want to understand why false positive results occur so they could be minimized.

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Some false postive prenatal genetic screens due to mother's extra DNA segments