After your eggs are retrieved as part of an IVF procedure, they are cultured in the lab for up to 7 days. The embryology team will check in on their growth periodically using a microscope. They look for certain signs that tell them how each embryo is developing. This post details what the embryology team is looking for in the final days of fertilization.
Embryo fertilization calendar following egg retrieval
Day 5, 6, and 7 are the days that the embryos can reach the blastocyst stage. Blastocysts are the goal stage for many IVF cycles because they can be biopsied for Preimplantation Genetic Testing(PGT) and, according to a study in the Journal of Human Reproduction, “blastocysts offer several advantages, the most important being synchronization of the embryos with the uterine endometrium and selection of the best quality embryos with a high implantation potential.”
Blastocyst embryos are made up of about 200 cells organized into 2 main parts: the inner cell mass, which will develop into the fetus, and the outer layer, known as the trophectoderm, that will develop into the placenta.
When PGT biopsies are done, a few cells are removed from the trophectoderm for testing.
Blastocyst stage embryos are also graded by the embryologist before they’re biopsied, frozen, or transferred. The grading applies to 3 components of the embryo:
Inner cell mass (A-C)
Expansion refers to the stage of growth that the blastocyst is in. The embryo is given a number that correlates with its stage:
Early Blastocyst (least expanded)
Blastocyst with small inner cavity
Full blastocyst with inner cavity that fills the embryo
Hatching expanded blastocyst
Fully hatched blastocyst
The higher the grade, the better quality the embryo.
The inner cell mass (ICM) is the portion of the blastocyst that will develop into the fetus. It’s graded with letters from A-C. A study in the Frontiers of Endocrinology explains the ICM grading like this:
“Many tightly packed cells”
“Many loosely grouped cells”
“Very few cells”
The best grading is A.
The trophectoderm is graded in a similar way to the ICM:
The best grade here is also an A.
Embryo grading can be complicated and overwhelming to look at and understand. The important thing to note is that even embryos with lower grades can still result in successful and healthy pregnancies.
PGT testing can shed more light on the likelihood of success for each embryo by examining its chromosomal makeup. If you plan to do PGT on your embryos, check out our article Everything You Need to Know About Genetic Carrier Screening and PGT to get all of the information you’ll need!
Note: All embryology labs function differently. Clinics may provide updates on different schedules. Make sure to ask your clinic when and how you will be updated regarding embryo development.
Zhao, Yan-Yu, Yang Yu, and Xiao-Wei Zhang. 2018. “Overall Blastocyst Quality, Trophectoderm Grade, and Inner Cell Mass Grade Predict Pregnancy Outcome in Euploid Blastocyst Transfer Cycles.” Chinese Medical Journal 131 (11): 1261–67. https://doi.org/10.4103/0366-6999.232808.