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Reducing False Positives in Prenatal Genetic Screenings

Prenatal testing is worrisome enough, now researchers have discovered a simple reason for many inaccurate results.

Image via pixabay user Skitterphoto

Prenatal screening in its present form is a developing technology, constantly being spurred onward by the demands of curious expectant mothers worldwide. What mothers may not realize is that while low-risk testing for abnormalities may be more credible than, say, a horoscope, sometimes the results can be unreliable.


Designed to determine the probability, or the risk, of a baby being born with certain genetic abnormalities that may lead to defects or disabilities like Down’s Syndrome, screening tests have been plagued since inception by the issue of false positives. No doctor, computer, blood, or urine sample can predict with absolute authority whether a baby will emerge healthy or not, and mothers are understandably hesitant to gamble on the life and health of their newborn.

According to researchers at the University of Washington in a study published Wednesday in the New England Journal of Medicine, the accuracy of prenatal tests may be compromised if the pregnant mother’s genome contains abnormalities in the usual number of specific DNA segments. In a way, however, every woman’s chromosomes develop uniquely.

"The approach used in the screening tests assumes that every woman carries the same proportion of genetic material on a given chromosome," explained the researchers in the study.

In reality, the idea of a “normal” set of chromosomes is arbitrary. Chromosomes vary slightly in composition and size from person to person, and the body sometimes deletes and duplicates certain regions of certain genomes as it sees fit. Longer chromosomes are subjected to this process more often than shorter chromosomes, and these longer chromosomes are sometimes the culprits that trigger a false positive result during screening.

A maternal duplication of a genome effectively increases the length of a chromosome on which it resides, potentially resulting in the false reading of an additional chromosome. Image via Wikimedia Commons

Modern prenatal genetic screens analyze the mother’s blood during pregnancy, which is a safer and less invasive alternative than sampling the fluid surrounding the fetus in the uterus. These blood tests are routinely offered to pregnant women of an older age, who are at greater risk of bearing a child with certain genetic abnormalities. The likelihood of a false positive increases as the pregnancy progresses, as well as with increasing maternal age. Screenings done between 10 and 18 weeks of pregnancy result in a 5 percent chance of false positives, which jumps up to 10-15 percent in screenings done between weeks 18 and 22.

“Currently, the causes of false positive results are poorly understood,” the researchers noted in the study. “Once you’re aware of the problem, you’re obligated to address it.”

Once the problem is properly identified, addressing it can be as simple as anticipating the extra genomes that will show up on tests and adjusting the results to account for it.

False positive screening results can have a profound psychological effect on an expectant mother, and may lead to unnessecary, riskier further testing. A better understanding of what causes false positives could save pregnant women and their families months of anxiety and uncertainty.