The secrets of the embryo selection process that no one has told you about
A team at an assisted reproduction laboratory looks through a microscope and evaluates tiny embryos. A crucial part of the surrogacy journey takes place here. They decide which embryos have the best chance of developing into a baby and which pose the greatest risks. This silent moment is key and relies on the well-known embryo selection process.
But how is an embryo identified as having “low quality”? What morphological and genetic criteria are used to discard or select them? In the following article, we will explain how this filter works, why it matters, and its relationship to the success of the surrogacy process.
Why does it matter to identify low-quality embryos in the embryo selection process?
In a surrogacy program, the intended parents want to give the best possible care to the baby. Part of that care involves ensuring that the transferred embryo has the best chance of implanting, growing well, and resulting in a healthy pregnancy.
The embryo selection process aims precisely at this: identifying, among all available embryos, those with the greatest potential for success, and reducing the risks of failure, miscarriage, or difficult pregnancy. This is achieved using morphological criteria (the embryo’s appearance, shape, number of cells, etc.) and genetic criteria (its chromosomal composition, the presence of aneuploidies, etc.).
When an embryo is described as “low quality,” it usually means that, according to certain criteria, it has a lower probability of implanting or resulting in a successful pregnancy. Understanding this difference helps in making informed decisions about surrogacy: which embryo to transfer, what to expect, and how to plan.
What do we mean by “low quality embryo”?
Let’s look at some nuances to explain it properly. In the context of the embryo selection process, a low-quality embryo is one that, when evaluated, does not optimally meet certain morphological and/or genetic parameters associated with good viability.
Morphological parameters
Experts observe the embryo at different times —for example, on day 3 and day 5 of culture— to assess its growth. They analyze the number of cells, the rate at which it divides, fragmentation (small pieces of cytoplasm that break off), the mass of internal cells —which will form the baby— and the outer layer, which will give rise to the placenta (the trophectoderm). They also assess how the blastocyst expands, that is, whether the embryo grows vigorously or not.
A recent meta-analysis showed that blastocyst morphology is key to predicting its quality: the trophectoderm has a greater impact on implantation than internal mass or the degree of expansion. Simply put, an embryo with few cells, significant fragmentation, or a poorly defined structure has a lower chance of implanting. In fact, according to a popular study, when fragmentation exceeds 25%, the probability of success decreases considerably.
Genetic parameters
But not everything is visible to the naked eye. Behind the embryo’s appearance lies something invisible: its genetic makeup. Thanks to techniques such as embryo biopsy and aneuploidy tests, specialists can determine if the embryo has the correct number of chromosomes or if it presents any abnormalities.
When an embryo has extra or missing chromosomes, the risk increases that it will not implant, that the pregnancy will not progress, or that genetic abnormalities will be present. In fact, the best way to classify embryos is by combining morphological and genetic criteria.
In summary, an embryo is considered to be of low quality when it presents an unfavorable morphology and/or genetic alterations that reduce its chances of development.
How is embryo quality assessed in the embryo selection process?
We will describe in a simple way the steps that are usually followed in the evaluation, within the context of surrogacy.
Embryo culture
After in vitro fertilization, the embryos are cultured in the laboratory for several days —usually until day 3, 5, or 6— so that they develop from just a few cells into a blastocyst (a more advanced stage of development). During this time, specialists observe them very closely, either with a traditional microscope or with incubators equipped with time-lapse video cameras, which allow them to monitor their development without interrupting their growth.
The most recent guidelines on assisted reproduction agree on one essential point: good embryo culture is the foundation for accurately assessing embryo quality. Without a stable and controlled environment, there can be no reliable evaluation.
Morphological evaluation
Embryologists analyze embryos following very specific and standardized criteria:
- Day 3: The number of cells is observed. Ideally, the embryo should have between 6 and 8 cells; if it has fewer, it may indicate slower development.
- Fragmentation: This assesses how much of the cytoplasm has broken off without forming complete cells. The less fragmentation, the better, as a “cleaner” embryo generally has a higher chance of implantation.
- blastocyst stage: here the criteria of the Gardner and Schoolcraft system are applied, which assesses three main aspects:
- Degree of expansion or hatching (how much the blastocyst has expanded).
- Quality of the inner cell mass (ICM), which will form the baby.
- Quality of the trophectoderm (TE), which will give rise to the placenta.
- A recent meta-analysis, with more than 42,000 embryos, concluded that the trophectoderm is the most determining factor in predicting a successful birth, followed by the ICM and then the degree of expansion.
- Other details: cell symmetry, the presence of vacuoles, and granulation are also examined. All these factors help to classify embryos, from those of the highest quality to those with the lowest potential.
The most recent ESHRE/ALPHA guidelines incorporate these criteria and establish them as an international reference for uniformly assessing embryo quality.
Genetic evaluation
In parallel or as a complement, some laboratories offer preimplantation genetic testing (such as PGT-A) to detect chromosomal abnormalities (aneuploidies). This part of the embryo selection process helps to discard embryos with a high risk of failure. According to one study, genetic evaluation improves selection, although always in combination with other parameters.
Ranking and transfer decision
Once the embryos have been evaluated, they are ranked according to quality (morphology and genetics) to decide which embryo to transfer first, which to freeze for later, or, in surrogacy cases, which one to give to the surrogate. ESHRE/ALPHA recommendations state that evaluation and classification must follow standardized criteria.
Specific considerations in surrogacy
In a surrogacy program, embryo quality is even more important because the goal is to optimize the cycle for both the intended parents and the surrogate. For example:
- Fewer embryos available → greater need for good selection criteria.
- Smaller margins of error: choosing an embryo with more potential can reduce time and cost.
- Information for prospective parents: understanding why an embryo is chosen and how it has been assessed.
What makes an embryo “low quality”?
When evaluating an embryo, specialists pay attention to a number of signs that may indicate lower quality. These clues arise from its appearance (morphological criteria), its genetic information, and the environment in which it develops.
From a morphological standpoint, an embryo can be considered of lower quality if, for example, on day 3 it only has four cells instead of the expected six or eight, or if its division rate is too slow. Fragmentation, the presence of small pieces of cytoplasm that have broken off, also plays a role: when it exceeds 25%, the chances of implantation are significantly reduced. Furthermore, embryologists observe whether the cells are symmetrical and uniform or if they exhibit differences in size and shape.
On days 5 and 6, when the embryo reaches the blastocyst stage, its degree of expansion, the quality of the inner cell mass (ICM) —which will form the baby— and the trophectoderm (TE) —which will give rise to the placenta— are also analyzed. Studies show that blastocysts with an “A” quality trophectoderm are much more likely to implant successfully. Other details, such as the presence of vacuoles, granulation, or an irregular zona pellucida, are evaluated with caution.
Regarding genetic markers, experts analyze whether the embryo presents aneuploidy, that is, an incorrect number of chromosomes, which can increase the risk of miscarriage or malformations. They also consider cases of mosaicism, where some cells are normal and others show abnormalities, a type of imbalance that is not always detected by observing morphology alone.
Finally, other factors influence embryo quality, although they don’t directly depend on the embryo itself: the age of the donor or the woman providing the eggs (older women have a higher risk of abnormalities), the quality of the laboratory, the culture conditions, the fertilization technique, and even the quality of the semen. In the case of surrogacy, the surrogate’s health and environment can also influence the final success of the process, even though they don’t affect the quality of the embryo itself.
What risks are involved in transferring a low-quality embryo?
The purpose of this entire embryo selection process is to reduce risks and optimize results. However, choosing a lower-quality embryo can increase certain risks, which you should be aware of:
- Lower implantation rate: a low-quality embryo is less likely to attach to the pregnant woman’s uterus.
- Higher probability of miscarriage or pregnancy failure.
- Higher risk of multiple pregnancy (if several embryos are transferred to compensate for lower quality) which carries more obstetric risks.
- Potentially higher emotional and economic costs: if the cycle does not come to a successful conclusion, it can be prolonged, generating stress and further intervention.
- In surrogacy, which is already a complex process, prioritizing quality helps to increase the likelihood of success.
Therefore, identifying low-quality embryos is not a stigmatizing issue, but a protective one: it allows for the reallocation of resources, better choices, and the setting of realistic expectations.
Can a “less good quality” embryo still result in a successful pregnancy?
Yes. It’s important not to fall into the trap of thinking that “low quality” means “impossible.” This distinction is key when supporting surrogacy.
- According to several studies, embryos with lower morphological scores can implant and develop into healthy babies. For example, the authors of an article in Human Reproduction indicate that, while morphology is useful, it has limitations and should be interpreted with caution.
- Extrachromosomal and environmental factors (uterus, pregnant woman, laboratory) play an important role.
- In some cases, when the “best” embryos are not available, it may be decided to transfer a lower-quality embryo with the consent of the prospective parents, after clearly explaining the risks.
- What is critical is that the medical team clearly explains the difference between “optimal quality” vs “acceptable quality,” and that expectant parents have clarity regarding their expectations.
In short: quality is a factor for success, not a curse. But choosing better is always more advantageous, especially in the context of surrogacy where every step takes on great importance.
How does the embryo selection process influence surrogacy?
The embryo selection process plays a key role in surrogacy, not only from a medical perspective, but also from an emotional and organizational one. Choosing well makes the difference between a shorter, more predictable path, or a longer one filled with uncertainties.
First, identifying and transferring higher-quality embryos helps reduce failed cycles. In surrogacy treatment, where each attempt involves a significant physical, emotional, and financial investment, increasing the chances of success on the first transfer can be a huge relief for the intended parents and the surrogate.
Furthermore, this process allows for a more targeted and safer transfer. The medical team and the intended parents usually choose to transfer the highest-quality embryo first, and it is almost always recommended to do so one at a time to reduce the risk of multiple pregnancies and associated complications.
Another important benefit is transparency. When prospective parents understand that the embryos have been evaluated using morphological and genetic criteria, they feel greater confidence and peace of mind. Knowing that decisions are based on objective data —and not on intuition— makes the entire process clearer and more humane.
There is also a direct impact on cycle planning and budgeting. Using a high-quality embryo allows for better time management, avoids unnecessary waiting, and reduces uncertainty. Furthermore, it helps ensure that everyone —the clinic, the surrogate, and the intended parents— is aligned and working with the same expectations.
Finally, this process encourages informed decisions. Parents can naturally ask how many embryos were created, how many meet the highest morphological criteria, whether genetic testing was performed, and what the quality ranking is. Having clear answers to these questions not only provides reassurance but also reinforces their sense of control and involvement at such an important time.
Good practices when choosing a surrogacy center with a focus on embryo selection
The first step is to contact your family counselor so they can provide you with information on embryo evaluation, both morphological and genetic. This makes a significant difference in the results. It’s advisable to ask which system they use to classify embryos —for example, the Gardner method or the updated ESHRE/ALPHA guidelines— as not all labs apply the same standards. If the lab offers or recommends preimplantation genetic testing (PGT), be sure to understand when it’s used and what kind of information it provides.
At Gestlife, we will always clearly explain how many embryos are typically obtained per cycle, how many are classified as high quality, and how many are discarded due to low quality —and, above all, why. We will also provide statistical results (implantation, pregnancy, and success rates per cycle).
Communication is another key point. In such delicate processes, understanding each step is fundamental to making confident and fearless decisions.
Finally, in surrogacy, everything works best when there is complete coordination between the Gestlife team, the surrogate, and the intended parents. The quality of the embryo is essential, yes, but the success of the process also depends on all parties working together, with clear information and mutual trust.
Illustrative example of how the embryo selection process is applied
Let’s share a simple case to illustrate the article:
Maria and David have opted for surrogacy. In their cycle, 6 embryos were generated. The laboratory team applies the embryo selection process:
- Morphological evaluation was performed on day 3: 4 of the embryos had 8 clean cells with little fragmentation, 2 had few cells or much fragmentation.
- The best-advanced blastocysts were labeled: of those 4, 2 had expansion 5, ICM A, TE A; the others had ICM B or TE C.
- They also decided to perform genetic testing to detect aneuploidies. Of the two with the best morphology, one was found to be euploid (correct chromosome number) and the other showed aneuploidy .
- Of the embryos with the least morphology, one was also euploid , another mosaic, another aneuploid , and another with no conclusive result.
- The team, along with Maria and David, decided to transfer first the embryo with optimal morphology and euploid genetics. The others were frozen as a backup.
- Thanks to this strategy, successful implantation and pregnancy were achieved. In this case, identifying “low-quality” embryos allowed the team to focus on the best one, optimizing time and resources.
This case, as an example, shows how morphological and genetic evaluation are combined in the embryo selection process within a surrogacy context.
Conclusion
The embryo selection process is one of the most important moments in any surrogacy program. Identifying lower-quality embryos—based on morphological and genetic criteria—is not simply about deciding which are “better” or “worse,” but about understanding how to increase the chances of success, reduce uncertainty, and make informed and responsible decisions.
Understanding what is assessed in morphology (such as cell count, fragmentation, and ICM and TE quality) and what is analyzed in genetics (such as aneuploidies and mosaicism) helps both prospective parents and the medical team work with greater clarity, confidence, and realistic expectations. A lower-quality embryo does not mean it cannot develop, but it does imply more factors to monitor and more careful planning.
If you’re starting your surrogacy journey, the best thing you can do is educate yourself and actively participate. Ask questions, listen, and understand each step of the embryo selection process. Knowing why one embryo is chosen and another discarded puts you in a position of control and peace of mind.
Because, in the end, this isn’t about finding the “perfect” embryo —which rarely exists— but about finding the best possible one for your story, with all the information at your fingertips and hope well supported.