The journey through in vitro fertilization (IVF) involves high stakes, particularly regarding the genetic health of an embryo. A central question is the probability that a developing embryo will be deemed chromosomally normal and suitable for transfer. This determination is made by Preimplantation Genetic Testing (PGT), a screening tool that assesses the chromosomal makeup of embryos before they are placed into the uterus. The test is performed on the blastocyst, a key developmental stage reached approximately five to six days after fertilization. Understanding the likelihood of a blastocyst yielding a genetically normal result, known as euploidy, provides necessary context for individuals navigating their fertility treatment options.
Defining Blastocysts and Genetic Testing
The term blastocyst refers to an embryo that has developed into a complex structure containing two distinct cell populations. The inner cell mass will ultimately form the fetus, while the outer layer, called the trophectoderm, will develop into the placenta and other supporting tissues. Embryologists typically perform genetic screening once the embryo reaches this five or six-day-old blastocyst stage. This timing is chosen because the embryo has grown large enough to safely allow for the removal of a small number of cells.
The most common screening is Preimplantation Genetic Testing for Aneuploidy, or PGT-A. The purpose of PGT-A is to count the number of chromosomes within the biopsied cells to confirm a normal complement of 46 chromosomes, defined as a euploid result. A euploid blastocyst is considered genetically normal and has the highest chance of leading to a successful pregnancy. The biopsy involves carefully removing about five to ten cells from the trophectoderm layer, a procedure that does not disturb the inner cell mass. The genetic material is then analyzed while the blastocyst remains cryopreserved, awaiting the test outcome.
Statistical Likelihood of a Successful Result
The percentage of blastocysts that pass genetic testing is not a fixed number but rather depends heavily on the age of the individual providing the eggs. This is the most important factor influencing the euploidy rate of a cohort of embryos. For individuals under 35 years old, the expected rate of euploid blastocysts is the highest, often falling in the range of 60% to 73% of all blastocysts biopsied. This means most blastocysts in this age group will have the correct number of chromosomes.
The likelihood of a blastocyst passing PGT-A begins to decrease steadily after age 35. For those between 35 and 37 years old, the euploidy rate drops to 50% to 54%. This reduction continues as age advances. Individuals aged 38 to 40 can expect a euploidy rate in the range of 38% to 41%.
The decline becomes more pronounced for older age groups, a phenomenon that highlights the impact of time on egg quality. For those aged 41 to 42, the percentage of blastocysts yielding a euploid result is often cited between 24% and 32%. By the time an individual reaches 43 and 44 years old, the euploidy rate can be as low as 17% to 24%. These figures represent population averages, and an individual’s results can vary widely based on their ovarian health and other factors.
Biological Factors Affecting Euploidy Rates
The primary reason for the variation in euploidy rates across different age groups is directly related to the health and quality of the oocyte. Errors in cell division, specifically during meiosis, are the leading cause of chromosomal abnormalities in the resulting embryo. Eggs are paused in meiosis for many years, and this extended period increases the chance of errors when the division completes upon ovulation and fertilization. This mechanism explains why increasing maternal age is the strongest predictor of a lower euploidy rate.
While the egg’s genetic contribution is the most influential factor, the blastocyst’s physical structure also plays a role. Embryos are graded based on the appearance of the inner cell mass and the trophectoderm, and better-graded blastocysts are more likely to be euploid. For instance, a high-quality trophectoderm grade is associated with a greater chance of a normal genetic result. Paternal age and sperm quality have also been linked to euploidy, though their influence is considered minor compared to maternal age.
Understanding Aneuploid and Mosaic Results
When a blastocyst does not pass genetic testing, the result falls into one of two categories: aneuploid or mosaic. An aneuploid result signifies that the cells tested have an abnormal number of chromosomes, such as an extra copy (trisomy) or a missing copy (monosomy). These embryos are typically not transferred because they are likely to result in implantation failure, miscarriage, or a genetic syndrome.
The second, more nuanced result is mosaicism, which occurs when the blastocyst contains a mixture of both euploid and aneuploid cells. Mosaic embryos represent a gray area because the genetic abnormality is not uniform across the entire embryo. This outcome complicates the decision-making process for patients and physicians.
Some mosaic embryos may still be considered for transfer, particularly those with a low percentage of abnormal cells or specific types of chromosomal errors. The decision to transfer a mosaic embryo is made on a case-by-case basis, often after genetic counseling to discuss the potential risks and prioritization compared to any available euploid embryos. This offers a more detailed understanding of the blastocyst’s potential.