Blastocyst – IVF terminology
As part of my fertility work and being an acupuncturist who has a keen interest in Fertility, Gynaecology and Obstetrics the term blastocyst often comes up as a question or a subject with my patients. This blog is written to help those trying to conceive especially where IVF is required, and a fertility clinic will often use the term blastocyst.
A blastocyst is a cluster of dividing cells made by a fertilised egg. It is the early stage of an embryo. A blastocyst is one step among many that lead to a pregnancy. A blastocyst forms about five to six days after a sperm fertilises an egg. Layers of cells in the blastocyst divide and separate.
What does a blastocyst look like?
Three days after fertilisation, a healthy embryo will contain about 6 to 10 cells. By the fifth or sixth day, the fertilised egg is known as a blastocyst — a rapidly dividing ball of cells. By day five or six the blastocyst will contain approximately 200 cells and have a complex structure in different areas. For IVF this complex cell structure is where grading is performed.
The internal cell mass (ICM) is a group of cells found inside the blastocyst which will eventually form the foetus. The trophectoderm (TE) is an epithelial cell layer that covers the blastocele and will form the placenta and amniotic membranes.
Grading Blastocysts
Fertility clinics use different measuring scales which does not make grading for IVF patients simple. It should be considered that grading embryos are about their stage of development.
Early blastocysts, with fewer cells are generally graded 1 or 2. This is because they have fewer cells and a poorly defined inner cell mass and trophectoderm. Blastocysts that are further along in their development are given an expansion grade of 3-6 where 6 will indicate an embryo that has completely hatched. You may find your clinic grades with the use of a letter A-F rather than a number, this again is regarding the most desirable set of cells and the embryo’s appearance.
This article written by Perez et al, regarding “How are embryo’s classified according to their day of development? is a fantastic read by a gynaecologist and embryologists on the development, quality of blastocysts as well as a number of questions which are on patient’s minds.
It is important to remember that the grade does not always reflect the true potential of an embryo. There are embryos that receive a less than perfect grade that do create beautiful babies, I have seen this in clinic. Grading is subjective and that IVF clinics do vary in how they assign grades to embryos and blastocysts.
