Non-Invasive Prenatal Testing
Non-Invasive Prenatal Testing
Human beings have 23 pairs of chromosomes with a total of 46 (two copies of each pair). Health and development issues appear if there is a missing or extra chromosome. When there is an additional copy of a chromosome (i.e. three copies instead of two), it is called a trisomy.
NACE is a complete prenatal test to detect anomalies in chromosomes 21, 18, and 13 (Down, Edwards, and Patau syndromes). It also detects the most common anomalies in the sexual chromosomes X and Y (Except for twin gestations).
NACE Plus has the same characteristics as the NACE test, but is an extended version that also incorporates the detection of chromosome 9 and 16 trisomies, which are closely related with spontaneous abortions in the first trimester, and identifies six microdeletions which are associated with major genetic syndromes.The NACE PLUS test is recommended for all women who want to rule out the presence of chromosomal abnormalities in the fetus without putting their pregnancy at risk. The test is especially suitable for women with a high risk of chromosomal abnormalities following 1st trimester screening (hormone analysis and ultrasound scan), or who have had a previous Down’s syndrome pregnancy.
Both tests detect foetuses with chromosomal anomalies with very high precision.
Aster IVF offers two genetic testing services for pregnant women.
The NACE test is valid in single and twin* gestations and NACE Plus is validated for single pregnancies.
*In these cases it does not provide information on the gender of the fetuses.
1. What are the differences with the anomalies detected by an amniocentesis?
At the chromosomal level, NACE detects anomalies in the number of chromosomes, not in their structure. It only detects anomalies for a limited number of chromosomes related with problems in gestations after the second trimester of pregnancy. NACE Plus also detects six syndromes related with microdeletions which are not detected with a conventional amniocentesis karyotype.
2. Can the study be done in twin gestations?
Only the NACE test, not the NACE PLUS test can be done, and bearing in mind that in these cases we cannot provide information on the fetal sexes, or about any alterations in sexual chromosomes; the test only informs about the presence or the absence of a Y chromosome.
3. Can the test be done in gestations with twin embolization syndrome?
If the sack has disappeared the NACE test can be indicated; however, it should be noted that DNA originating from the reabsorbed sac could remain in the maternal blood; this could result in a false positive regarding the chromosome number in the viable fetus.
If the sac remains (although it hasn’t developed) the NACE test should not be indicated. The probability of a false positive result after the sac has disappeared is very high, although there is still an elevated risk that the test could produce a false positive if the other fetus had disappeared not long before performing the test, given that quite a lot of DNA from the other fetus will still remain present.
Currently there isn’t any empirical data which can confirm the percentage risk or lowest levels of sensitivity of the test in these cases – it is a possible scenario because the DNA analyzed comes from the placenta.
6. What happens if a NACE or NACE Plus test gives a positive result?
An amniocentesis or chorionic villus biopsy should be carried out. Occasionally, in cases of arrested pregnancy conventional curettage is performed. The chromosomal analysis preformed depends on the alteration found and may be:
Conventional karyotype: which will serve to confirm/discard complete or partial aneuploidies in any chromosome as well as results suggestive of mosaicism.
Quantitative fluorescent polymerase chain reaction (QF-PCR): to discard/confirm aneuploidies of chromosomes 13, 18, 21, or sexual chromosomes.
Fluorescent in situ hybridization (FISH) and/or short tandem repeat (STR) analysis: to discard/confirm aneuploidies of chromosomes 9 or 16.
FISH and/or microarray-based comparative genomic hybridization (array CGH): to discard/confirm some microdeletions.
7. What happens if the fetal sex result from the NACE test isn’t the same as the ultrasound results?
A new maternal blood sample should be taken in a Streck tube and the genetic laboratories will determine the fetal sex (free of charge).
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