22 January 2020

Understanding what makes a healthy embryo

Understanding what makes a healthy embryo
Medical Director
Cesar Diaz-Garcia
MD PhD Assoc Prof
IVI London


Although there are many factors to take into account which determine a successful IVF cycle, one of the most important is the embryo quality. Around 60% of all miscarriages are caused by abnormalities in the embryo, therefore when having IVF it is advised to transfer the best quality embryo to increase your chances of getting pregnant. Our IVI embryologists play a fundamental role in the selection process when it comes to your embryos. This is where the process of embryo grading is used.


How Embryos Develop

In a natural pregnancy, the egg is fertilised by the sperm in the fallopian tube, dividing into multiple cells to form an embryo whilst making its way into the uterus to implant and continue to grow. In an IVF cycle, this whole process takes place in the lab until the embryo is ready to be transferred back into the mother’s womb. 


Embryos that have grown for three days in the lab are called cleavage stage embryos. However, at IVI we tend to allow your embryos to grow for five days before transfer and these embryos are called blastocysts. When the embryos are ready, the doctor will transfer the best quality embryo into your uterus. At IVI we work to a single embryo transfer policy with nearly all our patients. This is to avoid multiple pregnancies and the risks that come with it. Therefore, only one embryo will be transferred at a time, then, if all goes well, the embryo will implant into your uterus and grow into a baby.


Embryo Grading

The embryologists will monitor and grade the embryos to determine which ones have the best chance of being healthy and lead to a successful pregnancy. The embryologists look at the embryo growth rate, if the rate is too slow or too fast this can be a sign of chromosomal abnormalities. Grading of the embryos is a non-invasive method of selecting the best embryos for transfer.

The Gardner system of grading embryos at the blastocyst stage was developed in the 1990s and is now everyday practice for all IVI clinics. The embryo quality is reported using a number and 2 letter grades for the two different parts of each blastocyst (the inner cell mass, which is what later becomes the baby and the outer layer, which is what later becomes the placenta). The number, from 1 to 6, identifies how far the blastocyst has expanded and hatched out of its shell. The letters indicate the quality of the inner and outer cell mass. To put it into perspective an embryo with an A grade is excellent quality, B is good quality, C is fair quality, and D is poor quality.


In general, embryos that are very slow growing or stop growing altogether are unlikely to develop into a healthy embryo. Embryos graded AA are not that common and you’re much more likely to see B and C grades, but that doesn’t mean you have poor quality embryos as the grading method is not an exact science. This tool is used in addition to other factors to determine the best embryo for transfer. It is important to take into account that every patient is different, so when it comes to your treatment your doctor will also consider the patient’s age, fertility history, and other information to determine the best day of transfer. The embryologists will give you an honest assessment of the quality of your embryos and your chances for successful IVF treatment. 


At IVI, we transfer blastocysts on day 5 in nearly all cases, because only the best embryos will make it to that stage in the development process.


If you want more information about the process of embryo selection, why not visit the clinic during one of our bi-weekly open evenings where one of our embryologists will be able to answer all your questions. 

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