In recent years the possibility of screening for fetal trisomies through DNA analysis of the fetus in maternal blood has appeared. We now know that there are fragments of fetal DNA in maternal blood from the placenta, so through a peripheral maternal blood collection and processing by amplification and identification of fetal DNA we can analyze the fetal chromosomes. This method increases the detection rate of Down syndrome to more than 99% and reduces false positives to less than 0.1%. In addition, this test is capable of analyzing all chromosomes, although the maximum sensitivity and specificity has been demonstrated for Down syndrome.
Before its appearance, the only alternative to evaluate the risk of having a child with Down syndrome was the combined screening of the first trimester, which offers less sensitivity and specificity.
Therefore, fetal DNA analysis in maternal blood is the best screening method for trisomy 21. Ideally, it should be the method used in all pregnant women. However, due to its high cost, most health care providers only cover the cost for high-risk patients.
In general, we could say that this test would be especially recommended in the following groups of women:
- Women who underwent first trimester screening and are considered high-risk patients(≥1/270)
- Women who presented aneuploidies in chromosomes 21, 18 or 13 in their previous pregnancy.
- Women who are pregnant at ≥ 38 years.