One of the most complete prenatal tests is cell-free DNA (cfDNA) blood test, which detects the presence of Down, Edwards and Patau syndromes in a fetus (anomalies of chromosomes 21, 18 and 13), as well as the most frequent anomalies found in the sex chromosomes (X and Y).

As cell-free DNA circulates in the mother's bloodstream, this method requires only a sample of the woman's blood.

Mutation detection

Chromosomes are threadlike structures inside the nucleus of a cell, containing genetic information in the form of DNA. Each cell in the human body carries 23 pairs of chromosomes (46 in total). The first 22 pairs are numbered chromosomes, called autosomes, and the last pair determines the sex (two X chromosomes for women and an X and a Y chromosome for men). Any change either in the number of chromosomes or in the DNA they contain represents a genetic alteration. A presence of an additional chromosome is called trisomy. Genetic analysis is aimed at detecting such alterations.

During pregnancy, fetus DNA is released by the woman's placenta and circulates through the mother’s blood in small amounts from the 10th week onward in pregnancy. Recent advances in screening tests allow to measure the DNA from a certain chromosome. For example, Down syndrome implies an extra 21st chromosome, thus in case the disorder is present, higher than normal chromosome 21 DNA amount will be found in the sample.

Non-invasive prenatal tests do not pose any risks for either women or babies. Still, significant abnormalities can be detected before the baby is born. If the results are negative, it is unlikely that the baby will have Down syndrome or another trisomy disorder.

Cell-free fetal DNA blood tests are highly precise and reliable. This simple test of the mother's blood provides up to 99% accuracy in detecting trisomies, the most important of which include trisomy 21 (Down syndrome), trisomy 18 (Edwards syndrome), trisomy 13 (Patau syndrome) and the most common X and Y anomalies. Positive results should be followed by further tests to examine the state of the fetus and to exclude the probability of a false positive result. However, cfDNA tests are not as effective in women who are pregnant with more than one baby.

More advanced versions of the free DNA blood test may be performed to detect abnormalities of other chromosomes, primarily related to miscarriages in the first trimester, as well as to identify deletions, or loss of genetic material of chromosomes, which can also lead to serious genetic syndromes. These extended versions are recommended when ultrasound reveals certain abnormalities in the fetus.

Indications

• Women at the age of 35 or higher (mother's age is the primary risk factor for Down syndrome or other trisomy disorders),
• Women with high risk of chromosomal abnormalities revealed by first trimester screening (hormonal and ultrasound analysis),
• Women with a previous pregnancy with chromosomal abnormality,
• Women who wish to rule out the most frequent or severe chromosomal abnormalities.

Results

• Women who wish to rule out the most frequent chromosomal abnormalities.
• Women with high risk of chromosomal abnormalities after first trimester screening (hormonal and ultrasound analysis).
• Women with a previous pregnancy with chromosomal abnormality (e.g., Down syndrome).
• Women for whom ultrasound abnormalities have been detected that lead us to suspect a chromosomal abnormality.

Results

Using the latest sequencing technology to analyze fetal DNA allows accuracy of indication of Down syndrome as high as 99%. False-positive rate doesn't exceed 1% and typically amounts to 0.2%. Such tests present a high rate of informativity, since results are obtained in 99.9% of the analyzed samples.

Procedure

A cfDNA screening can be done starting from the 10th week of pregnancy. During a screening, a blood sample is taken from a vein (this usually takes less than 5 minutes) and sent to the laboratory. The study of chromosomes 21, 18, 13 and sex chromosomes is prompt and takes 3 working days. For more advanced version of the test, the results are available within 10 days. A health care professional will then explain you the results and what they might mean to you and your baby.