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False positives in prenatal testing lead to unnecessary worry

Parents tend to ask themselves numerous questions during prenatal care. Is it a boy or girl? Will the baby arrive early or late? During a pregnancy, parents look to medical professionals and established testing protocols to answer their single most important question: will my baby be healthy?

There are numerous tests available that have been designed to help doctors screen for chromosomal defects. Early detection of chromosomal abnormalities can help parents prepare for the challenges they might face in the future. However, the presence of false positives have lead many parents to worry needlessly and, ultimately, question the accuracy of the test itself.

A new study from researchers at Fred Hutchinson Cancer Research Center and the University of Washington was published in the New England Journal of Medicine. The researchers found a crucial defect in how these screening tests look at DNA. Quirks in the mother's DNA, for example, can lead parents to believe that their healthy baby actually suffers from a major disorder.

The analytical flaw lies in the fact that the test assumes that every woman has the same amount of DNA. Many women, in fact, have more DNA than would be considered normal due to harmless duplications of pieces of their chromosomes. By not accounting for this, the screening tests could very well find an abnormality in the fetus's DNA where one does not exist.

The researchers looked at four women who had received false positive results of a trisomy for chromosome 18 - a condition where a baby carries an extra copy of a chromosome and which can be fatal or severely debilitating. Of these four, two of the women were found to have the harmless duplication in their own DNA as noted earlier.

All four women went on to have healthy babies.

The researchers used computational models to show that this phenomenon might actually account for a significant number of the company's false positives. A screening test can only point to the likelihood that a condition exists, it is not meant to definitively diagnose a disease or disorder. However, the researchers believe that simple changes can reduce false positives and give parents the peace of mind they need.

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