How are low-quality embryos identified according to morphological and genetic criteria in selection?

Continuing from our previous article, up to half of the embryos generated in the laboratory may present some type of anomaly in their genetics or morphological development, which leads them to be considered “low quality” and not selected for transfer. This fact makes all the difference in any surrogacy or assisted reproduction protocol , where embryo selection is key.

In this article, we’ll explain how low-quality embryos are identified based on their morphological and genetic criteria, why this is so important in embryo transfer, and what the implications are for those considering surrogacy. At the end, you’ll find a frequently asked questions section addressing the most common concerns of intended parents.

Why does identifying low-quality embryos in surrogacy matter?

When we talk about surrogacy , we’re referring to a process in which intended parents depend on the selected embryo having a good chance of implanting, developing, and resulting in a healthy baby. If an embryo is of “low quality,” there may be:

• Lower implantation rate after embryo transfer .
• Higher risk of miscarriage or cycle failure.
• More cycles required, more costs and more stress for everyone involved.
• In surrogacy, these delays or failures have a greater emotional and logistical impact, because it is a more organized and planned process.

Therefore, understanding both the morphological and genetic criteria for identifying embryo quality is fundamental to optimizing the embryo transfer process and minimizing risks.

What does “low quality embryo” mean?

A low-quality embryo is one that, when evaluated, presents characteristics that indicate a lower probability of success: either due to its morphology (appearance, cell division, fragmentation, etc.) or due to its genetics (chromosomal abnormalities, etc.).

These abnormalities can prevent the embryo from implanting correctly, or they can increase the risk of miscarriage. For example:

• Poor cell division, many fragmentations, asymmetries in the cells.
• An embryo with an incorrect number of chromosomes ( aneuploidy ) or with cell mosaics.

Assisted reproduction laboratories use scoring or classification systems to distinguish the most promising embryos from those with less potential.

In the context of a surrogacy process, where IVF is often performed with the creation of embryos and then transfer to the surrogate’s uterus, this discrimination is key: the intended parents want to maximize the chances of success, and selecting a quality embryo is part of the process.

Morphological criteria for identifying low-quality embryos

The morphological evaluation of the embryo refers to how its cells look, how they are developing, whether they show signs of fragmentation or irregularities, and what stage of development they are at. Here I explain some of the main criteria:

Developmental age and stage

• blastocyst stage (day 5 or 6) has a better prognosis than one that does not.
• If the embryo takes too long to divide or exhibits slow cell division, it may be considered of lower quality.

Number of cells, symmetry, and fragmentation

• In the first few days (day 2-3) we assess how many cells it has, whether they are well distributed, whether they are of good size, and whether there is fragmentation (cell remains that do not integrate).
• If there are many fragmentations or many unequal cells, it is considered a worse prognosis.

Blastocyst evaluation : inner cell mass (ICM) and trophectoderm (TE)

When the embryo reaches the blastocyst stage , the morphology criteria change:

• Evaluate the inner cell mass (ICM) —which will become the baby— and the trophectoderm (TE) —which will become the placenta—. An embryo with a well-formed ICM and good quality TE indicates a better future.
• Blastocyst expansion is also analyzed (e.g., degree of zona pellucida expulsion , size, etc.). Studies show that certain combinations of morphological scores are associated with better live birth rates.
• For example, in the study that reviewed 42,974 embryos, it was found that the best combinations for live birth rate were among others “5AA, 4AA, 6AA…” with the least favorable being “1CC”.

When is morphological “quality” considered to be “low”?

An embryo can be considered to be of low morphological quality if:

• It has many fragmentations or severe fragmentation.
• The cells are asymmetrical or of unequal size.
• blastocyst stage (or it does so with a delay).
• The ICM or TE have “B” or “C” scores in rating systems that rank A as best.
• It has a slow or irregular development.

These embryos have a lower probability of implanting or leading to birth, so many centers discard them or use them as a less preferred option.

Limitations of morphological assessment?

• Although morphology provides a good guide, it does not detect genetic abnormalities in the embryo — an embryo can “look” good morphologically and have chromosomal defects.
• There is variability among laboratories in how scoring is performed and what exact criteria are used. For example, the ASEBIR system (Spain) has indicated a lack of universal consensus.
• New technologies such as “time- lapse ” allow us to see the evolution of the embryo in real time, but their clinical impact is still under study.

In the context of surrogacy and embryo transfer, morphology can be used to filter out low-quality embryos before considering other genetic analyses, thus optimizing the process.

Genetic criteria for identifying low-quality embryos

Beyond the form, the “genetics of the embryo” is a critical factor: an embryo may look good, but it may have chromosomal abnormalities that significantly reduce its chances of implantation or development into pregnancy.

What is genetically evaluated?

• Chromosome number abnormalities ( aneuploidies ): for example, too many or too few chromosomes. These abnormalities are associated with implantation failure and miscarriages.
• Structural chromosomal translocations, genetic imbalances, etc. In some cases, preimplantation genetic diagnosis (PGT) is performed for certain diseases.
• In the embryo selection process for transfer, a trophectoderm biopsy is performed at the blastocyst stage (day 5/6) and analysis is carried out using sequencing or microarray techniques to assess the genetics of the embryo.

Why is a genetically abnormal embryo considered to be of low quality?

Because these embryos have a higher probability of not implanting, or implanting but resulting in miscarriage and having possible abnormalities that prevent normal development.

For example, an embryo with aneuploidy may appear to have all the right characteristics, but its actual viability is lower. This is why the embryo’s genetics are considered key to selection.

Limitations of genetic analysis

• Not all laboratories offer the same types of tests (some may not detect mosaicism or certain rare abnormalities).
• The fact that an embryo is genetically “good” does not guarantee the success of the pregnancy: the pregnant woman’s uterine environment, endometrial preparation, the quality of the eggs/sperm, etc., still matter.
• There are additional costs and extra time associated with genetic analysis.
• Risk of “discarding” embryos that could have led to healthy babies, due to mosaicism or limitations of the technique.

In summary, the genetics of the embryo complements the morphology in the selection, and together they allow the identification of low-quality embryos that should not be transferred or should be considered with caution.

How are morphological and genetic criteria combined in the selection of embryos for transfer?

In modern assisted reproduction laboratories (and in surrogacy protocols), the selection of embryos for embryo transfer follows a route that combines both approaches:

1. Morphological evaluation : After fertilization, the embryos are cultured and observed daily or at each stage. Those with poor development, fragmentation, etc., are discarded or have their priority reduced. This is an initial morphological quality screening.
2. Culture to blastocyst stage : Many programs prefer to reach day 5/6, because this stage allows for better discrimination of viable embryos. An embryo that does not reach the blastocyst stage may be considered of lower quality.
3. Embryo biopsy and genetic analysis (if indicated): For embryos that pass the morphological filter, a trophectoderm biopsy and embryo genetic analysis can be performed to detect aneuploidies and other chromosomal problems.
4. Final selection for embryo transfer : The embryo with the best combination of morphology and genetics is chosen, reducing the risk of it being of low quality.
5. Transfer of embryos to the uterus of the surrogate mother, in the case of surrogacy, or to the uterus of the recipient mother.
6. Freezing of surplus embryos of good quality, for future transfers, or discarding of those of low quality according to protocol.

This combined approach significantly improves implantation rates, reduces the number of failed transfers, and optimizes outcomes. For example, one article notes that careful morphological assessment can increase the likelihood of implantation, although genetic evaluation of the embryo remains a key factor.

In surrogacy, where each transfer involves coordination, logistics, legal agreements and many parties involved, having a good selection of embryos — and avoiding transferring low-quality embryos — is especially relevant for efficiency, cost and emotional success.

When is it most critical to identify low-quality embryos?

There are situations in which the identification of low-quality embryos becomes more important:

• When intended parents are of an older age and egg quality may be compromised.
• When there is a history of multiple implantation failures or spontaneous abortions.
• In surrogacy processes where few embryos are available or a single embryo transfer is sought.
• If you want to reduce the risk of multiple pregnancy (important in surrogacy) and want the maximum probability of success per transfer.
• When parents want to ensure as much as possible before embryo transfer; that is, to minimize the risk of “the embryo being of low quality”.

In these cases, combining embryo morphology and genetics allows you to choose the embryo with the best prognosis, avoid wasting time, and optimize the emotional and economic investment.

What happens if a low-quality embryo is transferred?

If an embryo considered to be of low quality (morphologically or genetically) is transferred, the chances of success may be:

• Lower chances of implantation.
• Higher probability of miscarriage.
• Possible increased risk of complications.
• Need for more IVF/transfer cycles or that the surrogacy is not progressing as desired.

Conversely, avoiding the transfer of low-quality embryos can increase the efficiency of the process, which is vital when doing surrogacy, where every step has logistical and emotional implications.

Of course, selection does not eliminate all risks, as success also depends on other factors: the pregnant woman’s health, the endometrial response, the quality of the laboratory, etc.

Good practices to avoid low-quality embryos in surrogacy

For intended parents or clinics that perform surrogacy, these recommendations may be helpful:

• Choose an assisted reproduction center with experience, good rates, and that uses rigorous embryo evaluation protocols.
• Ensure that the laboratory evaluates embryo morphology and, if appropriate, embryo genetics.
• Consider the option of culturing to blastocyst (day 5/6) for better discrimination.
• Request clear information on how the center defines “low quality” embryos and what criteria it uses to discard or qualify them.
• Understanding the costs, timeframes, and possibilities of freezing good quality embryos.
• Ensure that the embryo transfer protocol is well coordinated with the surrogate or recipient mother, so that the highest quality embryo is ready for implantation at the optimal time.
• Have realistic expectations: even if the best embryos are selected, success is never 100% guaranteed.

In summary

In the process of surrogacy, the transfer of embryos is one of the most important moments.

• Selecting a good quality embryo — while avoiding low quality embryos — improves the chances of success, reduces costs, and reduces risks.
• The evaluation of the embryo’s morphology (how it looks, how it divides, how many cells, its symmetry, its development to blastocyst ) is the first filter to identify quality.
• The evaluation of the embryo’s genetics (chromosomal analysis, aneuploidies , mosaics) is a second filter, key to ensuring that there are no abnormalities that reduce viability.
• Combining both criteria allows for better selection, which is especially relevant in complex processes such as surrogacy.
• However, no system completely eliminates the risk of failure or complications: other factors also determine the final outcome of the pregnancy.

Frequently Asked Questions ( FAQs ) about low-quality embryos, morphology, embryo genetics, and surrogacy

Does a morphologically “bad” embryo always mean it will be of low quality?

Not always. Poor morphology indicates a lower probability of success, but it doesn’t guarantee that the embryo cannot be carried to term. However, in clinical practice, it is considered a lower priority for transfer.

If an embryo has good morphology, are genetic tests no longer necessary?

No. Good morphology is important, but it doesn’t rule out the presence of genetic abnormalities. That’s why many clinics combine morphological evaluation with genetic testing of the embryo for greater certainty.

Can the genetics of an embryo determine whether it is of low quality even if it appears morphologically good?

Yes. An embryo that looks visually “perfect” may have aneuploidies that reduce its viability. Therefore, embryo genetics is key to identifying “hidden” low-quality embryos.

Can embryos considered to be of low quality be “improved”?

Not exactly. If an embryo is classified as having low morphological or genetic quality, there is no “correction” to change that. What is done is to use the best ones for transfer and reserve or discard the others according to protocol.

In surrogacy, is the “best quality” embryo always transferred?

Ideally, yes. Surrogacy aims to maximize success; therefore , the embryo with the best combination of morphology and genetics is selected for transfer.

Does a “low-quality” embryo mean there can be no pregnancy at all?

Not always. Some lower-quality embryos can still lead to a successful pregnancy, but the likelihood is lower compared to high-quality embryos. It’s a matter of increased risk.

How many embryos do you need to be able to discard low-quality ones and choose high-quality ones?

There’s no universal minimum number, but the more embryos available (and of good quality), the greater the likelihood of selecting a high-quality embryo. If there are only one or two embryos, the selection becomes more critical, and the risk of them all being of lower quality is higher.