Firstly what is PGT-A testing?:
PGT-A evaluates the chromosome complement of the cells submitted for testing. Chromosomes are the structures that carry all of our genetic material.
Most individuals have 46 chromosomes in each cell of their body. Chromosomes are inherited in pairs; one copy of each pair comes from the egg and one copy of each pair comes from the sperm. The first 22 pairs of chromosomes (the autosomes) are the same in males and females. The 23rd pair of chromosomes are the sex chromosomes. Individuals with two X chromosomes are chromosomally female. Individuals with one X and one Y chromosome are chromosomally male.
PGT-A allows us to screen embryos for aneuploidy and select chromosomally normal embryos that are eligible for transfer. However, PGT-A alone does not increase the number of normal embryos available for transfer. Embryos are biopsied on days 5-7 of development (the blastocyst stage). A few cells from the trophectoderm are sent to a genetics lab for testing. PGT-A testing will then show us two possible results: euploid and aneuploid.
Pronounced “u-ploid,” euploid means the number of chromosomes observed in the embryo sample was correct at 46. Many times, embryo samples with euploid results will be referred to as “normal embryos.”
Pronounced “an-u-ploid,” aneuploid means the number of chromosomes observed in the embryo sample was incorrect. There may be extra or missing whole chromosomes, missing or extra pieces of chromosomes (see below), or a combination of these. Often, embryo samples with aneuploid results will be referred to as “abnormal embryos” or have “chromosomal abnormalities.”
There are nine different types of aneuploid results:
- Monosomy: mono- means one. In this case, it means one chromosome is present, rather than the expected two (remember, chromosomes are inherited in pairs). As an example, monosomy 7 means that only one copy of chromosome 7 is present in the tested cells instead of the expected two.
- Trisomy: tri- means three. This means three chromosomes are present, rather than the expected two, in the tested cells. A well-known example of a trisomy is Down syndrome. This is caused by the presence of three copies of chromosome 21, rather than two.
- Deletion: a missing piece of a chromosome, rather than an entire missing chromosome.
- Duplication: an extra piece of a chromosome, rather than an entire extra chromosome.
- Segmental aneuploidy: when a chromosome abnormality involves a portion of a chromosome, rather than the entire chromosome. As an example, if the tested cells are missing a portion of the top of chromosome 12, rather than the entire chromosome, this is a segmental aneuploidy.
- Haploid: a single set of chromosomes is present, rather than the expected two, (i.e., there are 23 chromosomes identified rather than 46).
- Triploid: a third set of chromosomes is present, rather than the expected two, (i.e., there are 69 chromosomes identified rather than 46).
- Polyploid: any chromosome result in which an entire extra set of chromosomes is present. Triploid (see above) is an example of a polyploid result, but an embryo sample with four (or more) sets of chromosomes is also considered polyploid.
- Mosaic: the presence of two or more populations of cells with different genetic compositions.1 For PGT-A specifically, mosaicism refers to two or more populations of cells with different chromosome complements.
The results of PGT are highly accurate; however, it is still considered a screening test. This means false positives and false negatives can occur. If you transfer an embryo tested by PGT, it is recommended you consider confirming the normal results through diagnostic prenatal testing such as chorionic villus sampling (CVS) or amniocentesis. Non-invasive prenatal testing (NIPT) can also be used as a first-line screen for a limited number of chromosome abnormalities during pregnancy. However, NIPT (like PGT) is also a screening test with the potential for false positives or false negatives.
Your fertility specialist may review these options with you in more detail as there are risks and limitations for each test that should be considered carefully.