The success of an Invitro Fertilization (IVF) cycle is affected by many factors with embryo quality as one of the most important. The majority of all miscarriages are caused by abnormalities in the embryos; therefore the transfer of quality embryos increases patient’s chances of successful pregnancy following IVF treatments.
In Fertility centres all over the world, the embryologists play an important role in embryo selection to be transferred to the uterus (womb).
What is an Embryo?
An embryo is an early stage of development, approximately two to eight weeks of fetal development after which it can then be called a fetus. Generally, in organisms that reproduce sexually, an embryo develops from a zygote, the single cell which results from the fertilization of the female egg cell by the male sperm cell. The zygote possesses half the DNA from each parent.
How Do Embryos Develop?
In a normal pregnancy, sperm fertilizes an egg as it travels down the fallopian tube. The cells in the fertilized egg divide and grow into an embryo as it moves to the uterus and it implant in the uterus to continue growing.
In an IVF cycle, fertilization and embryo development take place in the IVF laboratory. The eggs and sperm are combined and the fertilized eggs begin the process of dividing and growing. Embryos that have grown for three days in the laboratory are called cleavage stage embryos. Sometimes they are allowed to grow for five days before transfer and are called blastocysts. When the embryos are ready, a Specialists Gynaecologist/ Reproductive Endocrinologist will transfer the best quality embryo to the uterus. The number to be transferred is agreed upon before the procedure takes place by the patient and the clinician, usually no more than two embryos recommended. If it all goes well, one or two of the embryos will implant in the uterus and grow into a baby or babies except where there is an inevitable further split.
Embryo Grading
The embryologist monitors the embryos and prior to choosing which ones to transfer at the cleavage or blastocyst stage, grades them to determine which ones have the best chance of being healthy and resulting in a successful pregnancy and live birth. Embryo grading may be done at the cleavage stage or blastocyst stage. Embryo grading at the cleavage stage includes the number of appearance of the cells and the degree of fragmentation, which is the presence of small pieces of cellular material which have broken off during division of the cells. There are two different points of each blastocyst, inner cell mass or ICM which is what becomes the fetus and the outer layer, called the trophectoderm or TE, which becomes the placenta.
Embryo Grades and Chance of Pregnancy
One of the most common questions the embryologists gets from patients is ‘’how does the grades of embryos predict my chances of becoming pregnant’’. The objective of this write up is to explain how we grade embryos and what those grades mean as far as the embryos potential for development is concerned. All embryo grading systems are subjective, while we can make educated guesses about an embryo potential based on the experience of many embryologist grading, millions of embryos that look good physically may not tell us what is going on inside genetically.
Day 3 (Cleavage Stage) Embryo Grading: Quality of Division
Day 3 embryos are referred to as cleavage stage embryos. The reason for this designation is that the cells in the embryo are dividing or cleaving but the embryo itself is not growing inside. For example, think of a Pizza, when you slice it, you create more pieces of pizza but you don’t increase the size of the Pizza itself. This is what a cleavage stage embryo is like. The genetic material replicates and the cells divide, but the volume of embryo does not differ from the volume of the unfertilized egg. Conceptually, the division should be in a very specific sequence of one cell becoming two, two become four, and four become eight and so on. However, actual embryos do not divide synchronously we commonly see three, five, six cells, etc. This is not an indication of a poor embryo, but of one that is growing normally. Also, as embryos divides, small portions of the cytoplasm inside of the cells may break off and forms a bleb that we call fragments. They do not contain nuclei and are not complete cells. The cause of fragmentation is poorly understood, but embryos containing a lot of fragmentation are developmentally disadvantaged simply because the cells loose too much cytoplasm and thus become cellular machinery to the fragments.
Cleavage stage embryos are graded using two criteria: The number of cells in the embryos and their appearance under a high-power microscope. While the number of cells is objective, a good normally growing day 3 embryo will contain, between 6 to 10 cells (blastomere). We know that embryos that contain these numbers of cells are more likely to develop into viable blastocysts than embryo with fewer cells and are likely to result in better implantation and live births of healthy babies…………………………………………………..TO BE CONTINUED
