The journey through in vitro fertilization (IVF) often involves complex decisions, particularly selecting the best embryo for transfer. Preimplantation Genetic Testing for Aneuploidy (PGT-A) is a screening tool used to analyze the chromosomal makeup of embryos created during an IVF cycle. The purpose of this test is to identify embryos with the correct number of chromosomes, aiming to improve success rates and reduce the risk of miscarriage. PGT-A classifies embryos based on their chromosomal status, helping clinicians determine which embryos have the highest potential for a successful pregnancy. This process addresses a central question: what percentage of embryos will be considered viable after genetic screening?
Understanding PGT-A Screening
PGT-A is a genetic test that screens embryos for whole or partial missing or extra chromosomes, a condition known as aneuploidy. Embryos with the correct number of 46 chromosomes (23 pairs) are defined as euploid and are the preferred candidates for transfer. Conversely, embryos that fail the screening are classified as aneuploid, meaning they possess an abnormal number of chromosomes. Aneuploidy is a common reason for implantation failure, miscarriage, or the development of a genetic disorder.
The test is typically performed on a blastocyst, an embryo that has developed for five or six days after fertilization. At this stage, a small number of cells are carefully removed from the trophectoderm, the outer layer that will eventually form the placenta. The inner cell mass, which develops into the fetus, is left untouched. The biopsied cells are then analyzed in a specialized genetics laboratory.
The Baseline Percentage of Euploid Embryos
The overall percentage of embryos deemed euploid after PGT-A screening varies significantly, but general population studies offer a statistical baseline. Across a general IVF population, which includes a range of maternal ages, the rate of euploid embryos typically falls between 45% and 55%. This variability is influenced by patient demographics, specific testing methodologies, and embryo classification criteria.
This percentage represents a broad average across all patients undergoing testing. While factors like the total number of embryos available for biopsy can influence cycle-specific calculations, the largest factor driving statistical variability is the age of the patient supplying the eggs.
The Impact of Maternal Age on Euploidy Rates
Maternal age is the most significant factor influencing the rate of euploid embryos, demonstrating a clear inverse relationship with a woman’s age. This decline is primarily attributed to a decrease in egg quality and an increased likelihood of meiotic errors during cell division as the eggs age. The percentage of embryos that pass PGT-A screening drops sharply after the age of 35.
For women under 35 years old, the proportion of euploid blastocysts can be quite high, with rates often around 55% to 73%. The rate begins to decrease noticeably between the ages of 35 and 37, often falling to around 50%.
The decline becomes more pronounced as a woman enters her late 30s and early 40s. For women aged 38 to 40, the euploidy rate typically drops further, often to the range of 37% to 40%. By the age of 41 to 42, the chance of an embryo being euploid may be around 23% to 32%. For women over 42, the euploidy rate can fall to 17% or lower, with some data showing a rate around 5% to 12% for women over 43 to 45.
Aneuploid and Mosaic Embryo Classifications
Embryos that do not receive the euploid classification fall into one of two categories: aneuploid or mosaic. An aneuploid embryo is one where all the cells tested are predicted to have the wrong number of chromosomes. These embryos are generally not recommended for transfer because they have a very low probability of resulting in a successful pregnancy and often lead to miscarriage.
The second classification is the mosaic embryo, which represents a more complex result. Mosaicism means the embryo contains two or more distinct cell lines: some cells are euploid, and others are aneuploid. This result occurs when the chromosomal error happens after fertilization, during the early stages of cell division.
Clinical decisions regarding mosaic embryos depend on the level and type of mosaicism detected. Labs often classify mosaic embryos based on the percentage of abnormal cells, such as low-level (e.g., 20% to 40% abnormal cells) or high-level (e.g., 40% to 80% abnormal cells). Mosaic embryos, especially those with low-level mosaicism, are sometimes considered for transfer if no euploid embryos are available, as they have resulted in healthy live births. This decision involves careful genetic counseling, as transferring a mosaic embryo carries a potentially lower implantation rate and a slightly higher risk of miscarriage compared to a fully euploid embryo.