What cell-free DNA screening actually is
A blood test from you, reading fragments of placental DNA.
Fragments of DNA from the placenta circulate in your blood. From about 10 weeks, a blood sample can be analysed to estimate the chance of certain chromosomal conditions, most reliably Down syndrome (trisomy 21), and less reliably trisomy 18 and 13.
Pooled across 35 studies, it detects about 99.7% of trisomy 21 cases with a false-positive rate of 0.04%. Those are excellent numbers, and they are the ones marketed.
It also usually reports the baby's sex chromosomes, which is why many people take it for that reason alone. That is a legitimate reason. It is worth knowing that the same test may hand you information you did not ask for.
Where this comes from:
149. Gil MM, Accurti V, Santacruz B, Plana MN, Nicolaides KH. Analysis of cell-free DNA in maternal blood in screening for aneuploidies: updated meta-analysis. Ultrasound Obstet Gynecol. 2017;50(3):302-314.
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This page is educational. It supports the conversation with your own clinicians. It is not consent to any treatment, and it cannot assess you. If you are worried about a symptom now, see urgent warning signs.