Generally, patients have doubts about different topics related to the work that embryologists do, about their embryos, their development and their quality, as well as many other topics that are described below:

What is REM? Is it a decisive factor to perform one technique or another of Assisted Reproduction?

The REM allows us to know the value of sperm concentration and the percentage of sperm with progressive motility. It is not a decisive factor in exclusivity, but based on this valuable information, we will decide which assisted reproduction technique is the ideal one:

  • Artificial insemination (AI): if the REM shows good results, i.e. high concentration and good motility, we can perform AI. It is generally recommended to apply this technique when the REM is higher than 3 million progressive motile spermatozoa.
  • In vitro fertilization (IVF): if the capacitated semen sample has a progressive motile sperm concentration of less than 3 million, artificial insemination may not be effective and IVF will be recommended.

How many cells does an embryo have to have each passing day?

As we have said before, we count as day 0 the same day of the follicular puncture and therefore the day we perform the IVF/ICSI.

  • DAY 1: we will visualize FERTILIZATION, that is, if the union between the ovum and the sperm has occurred correctly. We say that an ovum has fertilized correctly when we see two nuclei (or pronuclei). One of them comes from the same ovum and the other from the sperm. These pronuclei contain DNA from both cells. When we get the news of a “bad fertilization”, it is because something has not gone as expected, i.e., they may have visualized a single pronucleus, more than two pronuclei (in cases of IVF) or no pronucleus.
  • DAY 2: the embryo should ideally have 4 cells at this time.
  • DAY 3: a perfect division would be that the embryo that we had with 4 cells, each one doubles, obtaining 8 cells.
  • DAY 4: this is the day when the embryo gives us the least information. It is a key moment in embryonic development since it must pass from one conformation (in cells) to a more complex one (morula). The cells of the embryo begin to “gather/merge” giving an appearance that provides little information to embryologists. It is the day that visually, it looks the worst.
  • DAY 5: the idea is that the embryo at this time begins to form a cavity to start the formation of the blastocyst. We can have from early blastocysts to expanded (fully formed) blastocysts. This moment is one of the most indicated to perform the embryo transfer, since if the embryo has reached blastocyst formation there is a high probability of implantation.
  • DAY 6: The embryo should be a fully formed blastocyst or hatching blastocyst, which means that the zona pellucida will be perforated and the interior of the embryo will begin to emerge and later implant in the maternal endometrium. Day 6 is the maximum day that embryos can be kept in culture in assisted reproduction laboratories. Embryos that reach day 6 at the blastocyst stage and with good quality should be transferred or vitrified.
embryonic development
Embryonic development. Source:

What does it mean for an embryo to be grade A, B or C?

The Spanish Fertility Society (SEF) and the Association for the Study of Reproductive Biology (ASEBIR) are two interest groups that are trying to unify the criteria for the classification of embryos so that biologists throughout Spain can evaluate embryos in a uniform way. This means that it proposes a classification based on the number of cells, number of fragments (cell debris), presence of vacuoles, symmetry and size of cells, etc. In short, according to a series of characteristics, we distinguish between 4 types of embryos:

  • GRADE A: embryo that has a number of cells according to the day of embryonic development in which it is. No fragmentation, or less than 11%.
  • GRADE B: has some cells more or less than normal and may have some fragments (11-25%).
  • GRADE C: 26 to 35% fragmentation and other organelles such as vacuoles may appear.
  • GRADE D: Very fragmented embryo, where cells can sometimes be mistaken for fragments.

This classification, together with the number of cells the embryo has on each day of development, can give us a prognosis of the success rate of transferring the embryo to the uterus.

embryo quality
Embryo quality. Source:

Can a C-quality embryo implant? What about A-grade?

Yes, you can implant. We gave it the letter C because of the pace of development and the way it looks visually. It has implantation power, but will have a somewhat lower percentage than A or B quality embryos.

What is the best day to make the transfer?

For embryologists, we say that the best day to perform the embryo transfer is day 5. This is due to the fact that, as we have had the embryo in culture for 5 days in the laboratory, we have been able to study in detail how it has been developing, its division speed, appearance of the cells, shape of the zona pellucida or membrane.

In addition, the passage from day 3 to day 4 is crucial for embryos. This is the time when the embryos begin to function as a unit and set in motion a series of mechanisms unique to each embryo. Here, unfortunately, many embryos are arrested because cell divisions do not continue. That is why if we transfer an embryo on day 3 and implantation does not occur we will never know if it was because the embryo has arrested or because of another factor.

In short, on day 5 we have more data about the embryo than on day 3.

Which is better to transfer 1 embryo or 2?

This decision is sometimes exclusively medical and sometimes very personal. It will depend on the circumstances and medical history of each patient. It is obvious that by transferring 2 embryos we increase the probability of implantation, but we also increase the risk of suffering a multiple pregnancy. This, depending on the circumstances, is inadvisable.

Given today’s technologies in reproductive laboratories and the excellent conditions of embryo culture in incubators, if we transfer a single embryo it is because we think it has implantation potential.

Is the embryoscope essential for my treatment?

No, it is not essential. The Embryoscope is an incubator that provides thousands of images that once joined together create a video in which the embryologist can view the division of the embryo. It is a very useful tool that helps us to select the embryo with the highest quality or implantation potential, but it is not essential.

Is embryo development the same in IVF as in ICSI?

Yes, the difference between one technique and the other is the way in which the sperm has fertilized the egg. In IVF, the egg is incubated with a high number of spermatozoa and the one that manages to cross the membrane of the egg will be the one that fertilizes it. Therefore, here there is no selection of the spermatozoon, but the one that is the most apt or that has had the best ovum-spermatozoon connection. This is the method that takes place naturally in the fallopian tubes at the moment a woman ovulates.

However, in ICSI it is something different. It is the embryologist who selects which sperm will enter the egg. This selection will be made taking into account their mobility and morphology. Once the “fittest” sperm is selected, it is introduced into the ovum with a thin glass pipette that passes through the membrane, leaving the sperm inside the ovum.

In the following days the development should be the same regardless of whether it is an IVF or ICSI embryo.

Why do I have to have ICSI and not IVF?

Performing one technique or another is, in principle, at the embryologist’s discretion, since the quality of the gametes will influence this decision. We recommend IVF in young patients with a good quality semen analysis and without pathologies associated with the gametes.

The ICSI technique should be used when we have a low number of oocytes, the semen is of low quality or poor morphology, when a Preimplantational Genetic Diagnosis has to be performed, or when there is a history of poor oocyte fertilization in previous cycles.

How is the sperm donor or egg donor selected?

In treatments that require a sperm donor, an egg donor or both gametes, the phenotype of the woman undergoing treatment is ALWAYS taken into account, and if she has a partner, the phenotype of the male is also taken into account. That is, donors are sought who are similar in ethnicity, skin color, eye color, hair color and shape, and in some cases may be requested to be of the same blood type as the couple or woman undergoing treatment.


E. gonzálvez, embryologist

Contact our fertility unit.

Vithas Perpetuo Internacional. Plaza del Doctor Gómez Ulla 15, in Alicante.

Phone: 965230397 / +34 606437458


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