A blastocyst is an early-stage embryo, a microscopic ball of cells that forms about five to six days after an egg is fertilized. It consists of an inner cell mass that will develop into the fetus and an outer layer of cells, the trophectoderm, which forms the placenta and other supportive tissues. For successful development, a blastocyst must be genetically normal, possessing the correct number of chromosomes. This article explores what genetic normality means for blastocysts, the factors influencing it, and the observed percentages in different contexts.
Understanding Blastocyst Genetic Normality
Genetic normality, or euploidy, refers to a blastocyst having the correct complement of 46 chromosomes, arranged in 23 pairs. When a blastocyst has an incorrect number of chromosomes, either missing or extra, it is considered genetically abnormal, or aneuploid.
Examples of aneuploidy include Trisomy 21, where there is an extra copy of chromosome 21, leading to Down syndrome. Other common trisomies are Trisomy 18 (Edwards syndrome) and Trisomy 13 (Patau syndrome), which often result in severe developmental issues. Aneuploidy can significantly hinder embryo development and is a primary cause of implantation failure, miscarriage, or the birth of a child with genetic disorders.
Factors Influencing Genetic Normality
The most significant factor influencing a blastocyst’s genetic normality is maternal age. As women age, the risk of aneuploidy in their eggs increases considerably, primarily due to issues during meiosis, the cell division process that forms eggs. For example, the rate of aneuploidy in embryos rises steadily from age 31, exceeding 50% for women over 40 and over 90% by age 45.
While maternal age is the predominant factor, some research has explored paternal factors, such as paternal age or semen parameters. However, studies generally indicate that male factors have a minimal influence on embryo aneuploidy rates compared to maternal age.
Observed Percentages and Their Implications
The percentage of genetically normal blastocysts varies widely, primarily depending on maternal age. For women under 35, the proportion of euploid blastocysts can range from approximately 50% to 70%. This percentage decreases sharply with advancing maternal age.
For women aged 35-36, the euploidy rate for good quality embryos drops to around 54%. This trend continues, with rates falling to about 37% for women aged 37-38, and further to 31% for those aged 39-40. For women aged 41-42, only about 23% of good quality embryos may be euploid, dropping to as low as 5% for ages 43-44, and potentially 0% for women aged 45-46.
These percentages have substantial implications for natural conception and assisted reproductive technologies like in vitro fertilization (IVF). In IVF, Preimplantation Genetic Testing for Aneuploidy (PGT-A) is used to screen embryos for chromosomal abnormalities before transfer. By identifying and selecting euploid embryos, PGT-A can significantly increase the pregnancy rate per embryo transfer and reduce the risk of miscarriage. While PGT-A improves the success rate of individual embryo transfers, it does not necessarily increase the overall cumulative live birth rates across multiple IVF cycles. This testing allows for the prioritization of the most viable embryos, leading to more efficient and successful fertility treatments.