In 2013, researchers reported an eye-opening case of a healthy pregnant woman with a puzzling prenatal test result. A routine genetic screen using cell-free DNA—a highly accurate blood test—suggested her fetus had an extra copy of chromosome 13 (Patau syndrome) and only one copy of chromosome 18. These results are devastating; both conditions can cause severe abnormalities. Those with Patau syndrome often only survive a few days or weeks after birth. But, when doctors looked at scans and did additional pregnancy testing, all they found was a healthy fetus developing normally. The woman carried on with her uncomplicated pregnancy and gave birth to a healthy baby.

The alarming genetic results may have been written off as a freak testing flub. But soon after giving birth, the otherwise healthy 37-year-old mother of two reported severe pelvic pain. Imaging revealed what looked like multiple bone tumors, and she was subsequently diagnosed with metastatic small cell carcinoma of vaginal origin. Tragically, she has since died.

Testing of one of her tumors found that the cancerous cells had an increased number of chromosome 13 relative to chromosome 18. Her prenatal test had picked up her deadly cancer.

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