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Blood-Based Tests of Fetal Sex: Are They Accurate?

Many expectant parents are impatient to learn if they are having a boy or a girl. So they may be attracted to new companies, like SneakPeek, which claim to be able to determine your baby’s sex with 99% accuracy early in the first trimester, with only a simple blood draw.

These new tests rely on what is known as cell free fetal DNA (cffDNA)–small fragments of DNA shed from the placenta into your bloodstream.

Unfortunately, according to a 2011 meta-analysis published in JAMA, some of these companies overstate the accuracy of their tests. The meta-analysis found that none of these cfDNA-based tests are accurate before 7 weeks. (The meta-analysis also examined urine-based tests. None of these were accurate.)

Between 7 and 20 weeks, accuracy for these blood-based tests ranged from 95-98%, depending on the specific techniques employed and the baby’s actual sex. (The tests were more accurate when they use a DNA amplification technique known as real-time quantitative polymerase chain reaction.) Only after 20 weeks were the blood-based tests 97-99% accurate.

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