How do embryologists select the best embryos?

The procedure of in vitro fertilization is complex and involves several important and necessary steps in order to increase the chances of success. The scientific personnel needs to be prepared for any last minute event that might arise as consequence of dealing with live cells. These gametes can show different behaviors at any moment during each hour they spend in the laboratory.


It has been said that the embryology laboratory is a surrogate environment resembling the same environment (in vitro) provided to the cells by the uterus inside the body (in vivo).


Once we obtain the eggs and make possible the fertilization with the sperm, in a few hours we will start having consecutive divisions that are crucial to the process of selecting the best embryos. First we need to take into account the fact that 16 to 18 hours after fertilization we need to pay attention to the formation of the pronuclei. Normal fertilization of the egg will show the presence of two pronuclei, not one, three, four, five, and so on. If we observe anything different than two together, symmetrical and centered then we need to suspect something is not right. This is what we should have:



And this is an example of an abnormal fertilization:




After this time we will observe subsequent divisions each day, starting from the early cleavage stage during the first 24 hours post-fertilisation, and day 2 and 3 embryos (morula) with 2, 4, 6, 8 and 16 cells).




Now the selection of the embryos is a key factor even for a well-trained embryologist. Involves the selection of the embryo/s that are considered stronger, with high quality and bigger chances of implantation, and the discarding or freezing of the ones that look less likely to succeed.


How to select the embryos if we have seen in our practice success with poor/low quality embryos developing into a healthy baby? And we have also seen not successful procedures using very good/high quality embryos? Not an easy task for us.


Some might be guided by their pure scientific knowledge than a good quality embryo it is likely to succeed rather than a slow, poor quality one. Some might use their embryologic instinct of using the embryo they consider to be stronger based on some of their qualities like good timing during fertilization and divisions, low fragmentation (rupture of the cells), cell symmetry, good zona pellucida, etc. There may be several factors to consider scientifically speaking. Also clinically it is important to judge the quality of the endometrium and the response of the embryo receptor to the treatment given to prepare for the embryo transfer. There are several factors to consider before selecting the best embryo for “the embryo transfer”. There is just one chance to make “the embryo transfer”, and even if the patient has remaining embryos to be used in future attempts, “the embryo transfer using those embryos” it will always be a one time chance. You will not have the same embryos or the same conditions to use them again. So, the embryologist criteria is really important to select the right “embryos to be transferred”. I call it “the point of no return”. Because there will just be one time to do it, and one time the same set of embryo/s to be used, not eh same again.


After being compelled to mention the scientific and clinical key points to be considered for the embryo transfer, among many others we will encounter during real life practice, it is important to mention another aspect to consider when selecting the right embryo/s, and is the behavior they showed during their short time stay on the embryology laboratory.





They will show an specific behavior, each one of them is a unique being with the capacity to communicate in their own way, and embryologists are the tool to translate their language.


During our training as embryologists we learn a lot about embryos, all the scientific and clinical based studies and facts, but we do not learn how to translate their language or read their behavior. I realized working with thousand of embryos that each one of them is unique and shows an specific way of communicating with us, and to help us choose among all of them which one/s is the most suitable for “the transfer” which will occur one time only. We do learn to master that knowledge and understanding on our day to day practice in the field.


When I choose the suitable embryo I share the responsibility with them, because they are in charge of showing me their behavior and signs of capacity to implant or not. There are also others factors to consider, however it seems they know them all, even the ones that don’t relate to the treatment like the psychological and spiritual condition of their parents. Some of my patients already know in advance when their procedure will be successful because somehow they have been communicating with their embryos through meditation and prays, and they let them know it is time. Some other patients undergo the difficult in vitro procedure in order to feel relieved because at least they tried it, even when they know the result is not going to be positive. Some other patients do the procedure knowing they need to be resilient because they will get the positive result in the second or third attempt. There are so many stories of patients feeling different things before, during and after their treatment, and some of those stories literally change the patient’s lives. When the embryologist works alongside the clinical and scientific team, and with the patients, the results can be astonishing.




I believe these tiny, microscopic beings, capable of behaving and sending messages to us through their short stay life on the laboratories, are changing the way medicine was before. They have been showing us life is possible in any form, in any way, at any moment and with any person. It is possible to get messages that we do not yet know where they are coming from and the reason they have been sending them in the first place. They have also been offered themselves for research to understand some of the mysteries of life and its many forms of changes throughout the centuries. Just decades ago it was nearly impossible to observe an image like this one, because life was considered to be sacred and just possible through the eyes of God. Today we have been allowed to admire these beautiful cells, which are accompanied by the being inhabiting it. They do deserve respect, consideration and understanding, because when I am able to create a baby after I saw him formed as an embryo before, and he is capable of recognizing my voice and respond to my touch, in a way like he knew me from before, it makes me believe there is certainly something else we have been missing out in our study of this science for decades, it might be time to check it out.