In vitro fertilization (IVF) involves intricate steps, with embryo selection being pivotal for a successful pregnancy. Two primary methods assess an embryo’s potential: morphological grading, which evaluates appearance, and genetic testing, which checks for chromosomal normalcy. A common question is why an embryo’s visual characteristics remain significant even after it’s confirmed to have the correct number of chromosomes. Understanding this interplay is important for those navigating IVF decisions.
What is Embryo Grading?
Embryo grading is a visual assessment performed by embryologists to evaluate the quality and developmental potential of embryos created during IVF. The Gardner grading system is widely adopted for blastocyst-stage embryos, which have developed for five to seven days and reached a complex stage with distinct cell populations.
The Gardner system assigns an alphanumeric score based on three key components. The first is the degree of blastocyst expansion (1-6), indicating how much the embryo has expanded and if it has begun to hatch from its outer shell. A higher number suggests more advanced development.
The second and third components assess the quality of two crucial cell groups: the inner cell mass (ICM) and the trophectoderm (TE). The ICM, which forms the fetus, is graded A, B, or C. Grade “A” signifies many tightly packed cells, while “C” indicates very few. Similarly, the TE, which forms the placenta, is also graded A, B, or C. An “A” grade means many cohesive cells, whereas “C” denotes a fragmented or irregular appearance.
What Does Euploid Mean?
In the context of embryos, “euploid” refers to an embryo that possesses a normal number of chromosomes, specifically 46 chromosomes arranged in 23 pairs. An embryo with an incorrect number of chromosomes, either missing or having extra, is termed “aneuploid.”
The determination of euploidy is achieved through a specialized genetic test called Preimplantation Genetic Testing for Aneuploidy (PGT-A). During PGT-A, a small number of cells, typically from the trophectoderm layer of a blastocyst, are carefully removed and sent for chromosomal analysis.
Euploidy is a significant factor for successful IVF outcomes. Transferring a euploid embryo is associated with a higher chance of successful implantation and a reduced risk of miscarriage. Conversely, aneuploid embryos are significantly more likely to result in implantation failure, pregnancy loss, or, in some cases, a baby born with a genetic condition if the pregnancy continues.
The Role of Grading in Euploid Embryo Selection
While confirming an embryo as euploid through PGT-A addresses the most common cause of implantation failure and miscarriage, morphological grading continues to provide additional information for embryo selection. Among euploid embryos, those with higher morphological grades generally exhibit better implantation, clinical pregnancy, and live birth rates compared to lower-grade euploid embryos. This suggests that while chromosomal normalcy is paramount, the visual quality of an embryo still offers predictive value.
The inner cell mass (ICM) grade, in particular, can be a strong predictor of pregnancy outcomes even among euploid embryos. For instance, euploid embryos with an ICM grade “A” have shown significantly higher odds of pregnancy and live birth compared to those with an ICM grade “C.” The trophectoderm (TE) grade can also influence outcomes, though some studies suggest its impact might be less pronounced than that of the ICM. This difference in success rates for euploid embryos of varying grades suggests that robust and efficient cellular processes, reflected in better morphology, may contribute to an embryo’s overall resilience and developmental potential.
A lower-grade euploid embryo still has a substantial chance of leading to a successful pregnancy. For example, some studies show that euploid embryos graded “BB” can still yield good chances of success, with pregnancy rates around 50% and live birth rates around 42.3%. Even euploid embryos with “C” grades in either the ICM or TE have resulted in successful pregnancies, though at lower rates compared to their higher-graded counterparts. This highlights that while grading provides a statistical advantage for higher-quality embryos, it does not disqualify lower-graded euploid embryos from successful development.
Navigating Embryo Selection Decisions
For individuals undergoing IVF, the combination of embryo grading and euploidy status offers comprehensive information to guide embryo transfer decisions. When multiple euploid embryos are available, morphological grading becomes a useful tool for prioritizing which embryo to transfer first. Clinics often recommend transferring the euploid embryo with the highest morphological grade, given its statistically improved chances of implantation and live birth.
However, it is important to understand that morphological grading is not an absolute predictor of success. A euploid embryo with a lower morphological grade can still lead to a healthy pregnancy and live birth. The visual assessment provides a valuable layer of additional information that complements the genetic screening results. This combined approach aims to optimize the chances of a successful outcome in IVF.