A euploid cell or embryo has the normal number of 46 chromosomes. This means it has an exact multiple of a single set of normal 23 unpaired chromosomes found in healthy sperm and egg cells. Euploid embryos have much greater chances of implanting successfully, higher chances of successful pregnancies, and reduced rates of miscarriage.
In contrast, an aneuploid embryo (or one with an abnormal number of chromosomes) accounts for increased rates of miscarriages, birth defects, and implantation failure.
FertilitySmarts explains Euploid
A human embryo goes through different stages of development until it can implant within the uterus, two of which are worth mentioning in this context. After fertilization, a single-celled zygote undergoes division around day 3. This process results in the formation of a 4-8 celled structure and is called the cleavage stage. The cleavage stage embryo continues to divide and enters the blastocyst stage on day 5. A blastocyst is a hollow structure containing a cluster of cells called the inner cell mass, which gives rise to the actual embryo.
Ensuring the development of a euploid embryo often requires the retrieval of at least 10 eggs. Generally, embryos from assisted reproductive technologies, such as in vitro fertilization (IVF) or intracytoplasmic sperm injection (ICSI), are placed into a woman’s uterus at either the cleavage stage (which is day 2 to 3 after egg collection) or the blastocyst stage (day 5 following egg retrieval). Typically, around 50% of the embryos proceed to the blastocyst stage at day 5, and about 50% of the day 5 embryos will be euploid.
Chromosomal aneuploidy occurs with greater frequency in women above 35-40 years of age. In order to prevent the transfer of aneuploid embryos, couples of this age undergoing IVF can opt for preimplantation genetic screening (PGS). PGS is a technique that can detect aneuploidy by sampling a cell from an IVF embryo. PGS analyzes the cell for a genetic disorder. It can be done on the day-3 "cleavage-stage" embryos or the day-5 "blastocyst-stage" embryos. This selective transfer of euploid embryos largely benefits women who are at risk for fetal aneuploidy, who have encountered recurrent pregnancy losses, or who have experienced multiple IVF failures.
Sometimes, comprehensive chromosome screening (CCS) is performed in place of PGS after the conventional IVF to analyze the 23 chromosome pairs in a day-5 blastocyst stage embryo. CCS is a form of PGS but it allows for a more detailed microscopic examination of all pairs of human chromosomes to ensure selective transfer of the healthiest euploid embryos.