A euploid embryo is one that possesses the correct number of chromosomes. This genetic integrity is a factor in an embryo’s potential for successful development. For those undergoing in vitro fertilization (IVF), understanding an embryo’s chromosomal status provides information that can influence treatment decisions. Selecting a euploid embryo for transfer is based on choosing one with a higher likelihood of leading to a healthy pregnancy.
Chromosomal Basics of Embryos
Human cells are built with a genetic blueprint of 46 chromosomes, organized into 23 pairs. One set of 23 chromosomes comes from the egg and the other set from the sperm. The term “euploidy” describes this normal state of having exactly 46 chromosomes, providing the genetic foundation for growth and development.
The opposite of euploidy is “aneuploidy,” a condition where an embryo has an incorrect number of chromosomes. A familiar example of a condition caused by aneuploidy is Down syndrome, which results from an extra copy of chromosome 21. The likelihood of producing aneuploid embryos increases with advancing maternal age, making chromosomal status a point of focus in fertility treatments.
The Process of Genetic Testing
To determine if an embryo is euploid, a procedure called Preimplantation Genetic Testing for Aneuploidy (PGT-A) is performed. This process begins after embryos are created through IVF and develop for five to seven days until they reach the blastocyst stage. At this point, the embryo consists of 70 to 150 cells organized into two main parts: the inner cell mass, which develops into the fetus, and the trophectoderm, which forms the placenta.
During the testing procedure, an embryologist performs a biopsy, removing a small sample of about three to ten cells from the trophectoderm layer. This biopsy is designed to be minimally invasive, leaving the inner cell mass undisturbed. Following the biopsy, the embryos are vitrified, a fast-freezing technique, and stored while the sampled cells are sent to a genetics laboratory for analysis.
At the lab, technicians use methods like next-generation sequencing (NGS) to count the chromosomes in the biopsied cells. This analysis reveals whether the embryo has the normal 46 chromosomes (euploid) or an abnormal number (aneuploid). The results typically take a few weeks to process, after which a chromosomally normal embryo can be selected for future transfer.
Impact on IVF Outcomes
The primary objective of using PGT-A is to improve the chances of a successful pregnancy through IVF. By selectively transferring a euploid embryo, fertility specialists aim to increase the likelihood of implantation and reduce the risk of miscarriage. Since aneuploidy is a major contributor to both implantation failure and early pregnancy loss, avoiding the transfer of aneuploid embryos can streamline the path to a viable pregnancy.
Clinical data suggests that transferring a chromosomally normal embryo can lead to higher pregnancy rates. Some research indicates that the implantation rate for a single euploid embryo transfer can be around 69% for the first attempt. One study focusing on patients with up to three euploid embryo transfers found that nearly 95% achieved a pregnancy.
Because PGT-A provides information about an embryo’s chromosomal health, it supports the practice of a single embryo transfer (SET). Transferring a single, healthy embryo reduces the risks associated with multiple pregnancies, such as premature birth. This approach focuses on the goal of one healthy baby at a time. The age of the person providing the eggs does not appear to affect the success rate once a euploid embryo is transferred, as the chromosomal normality is the more influential factor.
Testing Limitations and Nuances
PGT-A has limitations and complexities, including the existence of “mosaic” embryos, which contain a mixture of both euploid (normal) and aneuploid (abnormal) cells. These embryos present a challenge, as their developmental potential is not always clear. Clinics and geneticists have established criteria to classify mosaicism, often categorizing embryos as low-level or high-level mosaic based on the percentage of abnormal cells found in the biopsy. Transferring a mosaic embryo may be considered if no fully euploid embryos are available.
Another nuance is that the PGT-A test analyzes only a small sample of cells from the trophectoderm, which becomes the placenta, not the inner cell mass that forms the fetus. There is a possibility that the biopsy result may not perfectly represent the entire embryo. Studies have shown that re-testing embryos can sometimes yield different results, highlighting that a single biopsy is not always definitive.
The transfer of a euploid embryo does not guarantee a live birth. A successful pregnancy is a complex process influenced by many factors beyond chromosome count, including the health of the uterus and other genetic or developmental aspects of the embryo not assessed by PGT-A. The test screens for the number of chromosomes but does not detect single-gene disorders.