FAQ: Understanding Embryo Development and Quality
December 24, 2020
For years, in vitro fertilization (IVF) has been used to allow infertile couples to have a baby. Although this technique is effective, it requires precision when embryos are selected for transfer or freezing. How to choose those that are likely to be implanted in the future mother's uterus?
Evaluation of oocytes and embryos
At each stage of IVF, oocytes and embryos are examined with the utmost care. To optimize the chances of pregnancy, their morphological aspect and development are observed.
The fertilized oocytes are observed individually on days 1, 2, 3 and 5 after fertilization. If the development of the embryos is slower than expected, the transfer can be postponed until day 6.
Day 1. The embryologist examines the fertilization of the egg. The fertilized egg is designated by 2pn (pronuclei, number of cells), the unfertilized egg by 0pn. The fertilized egg with abnormalities is designated by 3pn, the unfertilized egg by 0pn.
Day 2. An embryo on day 2 should ideally consist of 4 cells.
Day 3. At this stage, embryos should consist of 6 to 12 equal blastomeres. If their morphology conforms, the embryos will be graded with an “a”. For example, 8a denotes 8 equal blastomeres.
Sometimes some blastomeres decompose by forming bubbles. This process, called fragmentation, prevents the correct division of the cells of the embryo and can lead to the development of embryos with a low capacity to implant in the maternal uterus. If there are few fragments, the embryo is evaluated with a “b”, but if the fragmentation takes up most of the embryo, a “c” or a “d” is placed.
Day 5. On day 5, the embryos, called blastocysts, must be composed of more than 64 cells. Two letters are used for their evaluation. The first letter is the number of cells in the embryo (A, B, C, D). The second is the number of cells in the placenta (A, B, C, D).
Grade A and B embryos are considered to be of good quality with a high chance of implantation. Grade C means that the embryo is of intermediate quality, grade D shows that the embryos are of poor quality, which greatly reduces the chances of implantation.
Blastocyst evaluation figures only indicate the stage of their development (1 to 6) and do not make any assumptions about their quality and potential for implantation.