Cleavage Stage Embryos, What happens on Day 3 of an IVF Cycle? We take the embryos out of the incubator to feed them (change the media they are growing in) and check on them. We give them grades at this time and update the physician on the status of the cleavage stage embryos. In the IVF Lab, whenever we move eggs, sperm, or embryos to a new dish, the procedure has to be double witnessed. That means, that name, date of birth, and medical record number are confirmed to be the same on the dish they come from, to the cultural dish they are moving to. An electronic witnessing system may create a record of this movement.
By Day 3 of an IVF cycle, we know how many embryos are “cleaving”. We will know if “0PN” and 1PN embryos (see our Embryologist perspective on Day 1: Fertilization Check). have started dividing, or if any of the normally fertilized eggs “arrests” at the 1 cell stage. Cleaving means the cells are dividing symmetrically and exponentially. One cell should become two, and should become 4 and 4 should become 8, 8 should become 16. Day 3 embryo grading systems are based on counting the number of cells. The thought has always been that an even cell number was better than an odd cell number, and that embryos should have between 4 and 8 cells on day 3. Most day 3 embryo grading systems also give a grade for symmetry and fragmentation, and other subjective variables like the the quality and clarity of the cell’s cytoplasm.
Embryo fragmentation is a fairly common occurrence. It happens when there is an uneven division of the cells of the embryo. These fragments are of no use to the embryo and are considered “junk” pieces of cytoplasm. The higher the degree of fragmentation, the lower the likelihood of pregnancy. However, fragmentation of 0-15 percent is quite normal and those embryos typically do well.
The standard of care in the US is to grow embryos to blastocyst stage and freeze all of them, and prepare the uterus in a separate cycle. Many embryos look good on day 3, but do not go on to become blastocysts. By day 5, you will know what stage they are and that will give you a much better indication of their ability to make a pregnancy. Many clinics in the US have stopped giving day 3 grades to patients because of the stress and anxiety it causes, and the low amount of information contained in those grades.
On day 3, something happens called the embryonic genome activation. Up until day 3, the embryo has been nurtured by factors found in the egg cell. By day 3, the name part of the genome has to kick on. This is exactly when we see a “4 cell block” – the embryonic genome often cannot progress. The embryonic genome activation is a process during which the embryonic genome is activated, i.e. when transcription is evident (day 3 of human embryo development, at the 4- to 8-cell stage). Human embryo development begins in relative transcriptional silence with an oocyte to embryo transition that lasts for about 3 days.
Assisted hatching is where an opening is created in the zona pellucida (the outer coating of proteins) of an embryo. Many IVF labs hatch embryos on day 3 in order to create a little opening that some cells of the trophectoderm to grow out of, where they can be snipped off safely and sent out for genetic analysis. Assisted hatching can also be performed on other days and for other reasons.
Mosaicism can start to be detected by day 3, and any chromosomal division that is abnormal at this stage can lead to a significant proportion of cells being abnormal by the blastocyst stage. One blastomere used to be removed at Day 3 for genetic analysis, but tis type of biopsy is no longer common.
