Navigating in vitro fertilization (IVF) involves decisions centered on embryo quality. Preimplantation genetic testing (PGT) screens embryos for genetic conditions before transfer to the uterus. PGT provides an assessment of an embryo’s genetic health. Understanding the statistics associated with this testing is vital, as it directly influences the likelihood of a successful pregnancy and live birth.
Understanding Preimplantation Genetic Testing
Preimplantation Genetic Testing (PGT) is a specialized procedure performed on embryos created through IVF to analyze their genetic material. Testing occurs when the embryo has developed into a blastocyst, typically five to seven days after fertilization. The process involves a delicate biopsy where a few cells are sampled from the trophectoderm, the outer layer that will eventually form the placenta.
The sampled cells are sent to a specialized genetics laboratory for analysis. PGT is categorized into three main types based on the screening focus. PGT-A screens for aneuploidy, which is an abnormal number of chromosomes. PGT-M is used for known risks of specific single-gene disorders, such as cystic fibrosis. PGT-SR detects structural chromosomal rearrangements, like translocations, carried by a parent.
Statistical Expectations for Embryo Viability
The number of embryos that pass genetic testing is primarily determined by the age of the egg provider. On average, roughly half of all embryos tested through PGT-A are chromosomally normal, or euploid, across all age groups. This average is misleading because the percentage of euploid embryos decreases sharply as maternal age increases, reflecting a higher likelihood of errors in the egg’s meiotic division.
For women under 35, the euploidy rate is highest, typically ranging from 55% to 70% of blastocysts being chromosomally normal. As age progresses, the percentage of viable embryos drops significantly. Women aged 38 to 40 often see euploidy rates fall to approximately 35% to 40% of tested embryos.
The decline becomes more pronounced for women over 40, where the percentage of embryos that pass PGT-A can be as low as 20% to 25%. For women 43 or older, the chance of an embryo being euploid may be less than 10%, sometimes resulting in zero euploid embryos in a cycle. Passing the test means an embryo is classified as euploid, possessing the correct 46 chromosomes. The overall success rate is overwhelmingly dependent on the maternal age at the time of egg retrieval.
Primary Reasons Embryos Do Not Pass Genetic Testing
The main biological reason embryos fail the PGT-A screen is aneuploidy, which is an incorrect number of chromosomes (either too many or too few). These numerical errors typically arise during meiosis, the formation process of the egg or sperm. Aneuploidy is the most common genetic cause of implantation failure and miscarriage.
Most aneuploid embryos are considered non-viable and are not recommended for transfer. Embryos may also fail due to mosaicism, a condition where the embryo contains both euploid and aneuploid cell lines. Mosaicism occurs after fertilization during the embryo’s early cell divisions. The clinical handling of mosaic embryos is complex and depends on the percentage and type of abnormal cells detected.
Embryos may also fail the PGT-M or PGT-SR tests. PGT-M failure means the embryo carries the specific single-gene disorder the parents sought to avoid. PGT-SR failure indicates the embryo inherited a structural chromosomal abnormality, such as a large deletion or duplication, from a parent. While aneuploidy dominates PGT-A failure, the specific diagnosis dictates the failure mechanism for the other PGT types.
Patient-Specific Factors Influencing Success Rates
Beyond maternal age, several patient-specific variables influence the final number of viable embryos. Sperm health and quality play a role, as male factor issues like poor morphology or high DNA fragmentation are associated with increased aneuploidy rates. Although the egg contributes the vast majority of chromosomal errors, the paternal contribution is not insignificant. Ovarian reserve and response to stimulation medications are also relevant, determining the total number of eggs retrieved in an IVF cycle.
A higher number of retrieved eggs generally translates to a larger pool of embryos, increasing the probability of finding a euploid one, especially for older patients. Prior reproductive history, such as recurrent pregnancy loss or failed IVF cycles, can signal underlying issues that predispose a patient to lower euploidy rates. Furthermore, specific protocols and laboratory conditions impact development, influencing how many fertilized eggs reach the blastocyst stage necessary for PGT. The patient’s overall health status, including factors like body mass index, can subtly shift results above or below expected statistical averages.