Preimplantation Genetic Testing for Aneuploidy (PGT-A) is a procedure used during in vitro fertilization (IVF) to analyze the chromosome count in an embryo before transfer. The goal is to identify embryos with the correct number of chromosomes, helping select the one most likely to result in a successful pregnancy. PGT-A involves taking a small biopsy of cells from the trophectoderm, the outer layer of the blastocyst that will eventually form the placenta. Understanding the report requires knowing the fundamental genetic terms that define the embryo’s status.
The Genetic Basis of PGT-A Results
The purpose of PGT-A is to evaluate for aneuploidy, which describes an abnormal number of chromosomes in a cell. Human cells typically contain 46 chromosomes, arranged in 23 pairs, a condition known as euploidy. The first 22 pairs are autosomes, and the final pair consists of the sex chromosomes (XX or XY). Euploid embryos are confirmed to have this correct count of 46 chromosomes. An aneuploid cell, conversely, has either too many or too few chromosomes. This numerical error is a common cause of implantation failure, miscarriage, and some genetic conditions. PGT-A allows clinicians to prioritize embryos with the highest probability of viability.
Decoding the Primary Result Classifications
When your PGT-A report arrives, the overall classification assigned to each tested embryo will fall into three main categories: Euploid, Aneuploid, and Mosaic.
The Euploid designation is the most favorable result, indicating the tested cells contain the expected 46 chromosomes. These embryos are prioritized for transfer as they are the most likely to implant and develop into a healthy pregnancy.
The term Aneuploid means a whole chromosome or a significant segment is missing or extra, and this abnormality is present in the majority of the biopsied cells. Aneuploid embryos are generally not recommended for transfer due to the high risk of implantation failure or miscarriage. Errors in chromosome number are the most frequent genetic abnormality in human embryos.
The third result is Mosaic, indicating the embryo contains a mixture of both euploid (normal) and aneuploid (abnormal) cells. This arises from an error in cell division after fertilization, resulting in two or more distinct cell lines. Mosaicism is categorized based on the percentage of abnormal cells detected, often ranging from 20% to 80% aneuploid content. Transfer of mosaic embryos is sometimes considered when no euploid embryos are available, but it requires careful consultation due to higher risks compared to a euploid transfer.
Interpreting Specific Chromosomal Findings
When an embryo is classified as Aneuploid or Mosaic, the report provides specific shorthand to identify the precise chromosomal error. Whole chromosome errors are described using terms like Trisomy or Monosomy, followed by the affected chromosome number. Trisomy means there are three copies of a specific chromosome instead of two, represented by a plus sign (e.g., +21 or +13). Monosomy means only one copy is present, indicated by a minus sign (e.g., -7).
Reports may also detail Segmental Aneuploidy, where the abnormality affects only a portion of a chromosome. This is denoted by “deletion” (del) or “duplication” (dup), indicating a missing or extra piece of a chromosome arm. For example, del(5q) signifies a deletion on the long arm (‘q’ arm) of chromosome 5.
The term Complex Aneuploidy signifies that multiple chromosomes are affected by errors. Embryos with complex aneuploidy have a very low potential for a successful live birth outcome. For mosaic embryos, the report specifies the percentage of aneuploid cells and the affected chromosome, such as “mos +15 (35%)” for mosaic trisomy 15 with 35% abnormal cells.
Clinical Decisions Following PGT-A Results
The PGT-A report guides the prioritization of embryos, but the final decision is made in consultation with your reproductive endocrinologist and a genetic counselor. Euploid embryos are always given the highest priority for transfer due to their high probability of success. Transferring a mosaic embryo is typically reserved for cases where no euploid embryos are available.
The priority among mosaic embryos is determined by the percentage of abnormal cells and the specific chromosome involved. Low-level mosaicism (a lower percentage of abnormal cells) and mosaicism involving certain chromosomes are more favorable than high-level mosaicism or complex errors. Genetic counseling is necessary when considering a mosaic embryo transfer to fully understand the specific risks and potential outcomes.
For any transferred mosaic embryo, additional prenatal diagnostic testing is usually recommended during pregnancy. Procedures like chorionic villus sampling (CVS) or amniocentesis can confirm the chromosomal status of the fetus, as PGT-A only analyzed cells from the placental tissue. It is important to remember that PGT-A only screens for chromosome number and does not test for single-gene disorders, which require PGT-M testing.