Hi Sharkey,
The below extract was posted by another person who replied to someone with similar concerns as you:
Human blastocyst morphological quality is significantly improved in embryos classified as fast on day 3 (>or=10 cells), bringing into question current embryological dogma.
Luna M, Copperman AB, Duke M, Ezcurra D, Sandler B, Barritt J.
Mount Sinai School of Medicine, Department of Obstetrics and Gynecology, New York, New York 10022, USA.
Comment in:Fertil Steril. 2008 Jul;90(1):243-4; author reply 244-5.
Abstract
OBJECTIVE: To evaluate developmental potential of fast cleaving day 3 embryos. DESIGN: Retrospective analysis. SETTING: Academic reproductive center. PATIENT(S): Three thousand five hundred twenty-nine embryos. INTERVENTION(S): Day 3 embryos were classified according to cell number: slow cleaving: <or=6 cells, intermediate cleaving: 7-9 cells, and fast cleaving: >or=10 cells, and further evaluated on day 5. The preimplantation genetic diagnosis (PGD) results of 43 fast cleaving embryos were correlated to blastocyst formation. Clinical outcomes of transfers involving only fast cleaving embryos (n = 4) were evaluated. MAIN OUTCOME MEASURE(S): Blastocyst morphology correlated to day 3 blastomere number. Relationship between euploidy and blastocyst formation of fast cleaving embryos. Implantation, pregnancy (PR), and birth rates resulting from fast embryo transfers. RESULT(S): Blastocyst formation rate was significantly greater in the intermediate cleaving (72.7%) and fast cleaving (54.2%) groups when compared to the slow cleaving group (38%). Highest quality blastocysts were formed significantly more often in the fast cleaving group. Twenty fast cleaving embryos that underwent PGD, formed blastocysts, of which 45% (9/20) were diagnosed as euploid. Aneuploidy was diagnosed in 82.6% (19/23) of arrested embryos. A 50% implantation and 100% PR and birth rate were achieved with embryo transfers involving fast cleaving embryos. CONCLUSION(S): Fast cleaving embryos not only reach the blastocyst stage at a similar rate to intermediate cleaving embryos, but also exceed morphological quality criteria on day 5. Fast cleaving embryo transfers demonstrated a high clinical potential.
I also found this extract from https://fertilitylabinsider.com/2010/11/embryo-stages-progression/:
What is significant at cleavage stage? Embryologists have long looked for characteristics at this stage which will identify which embryos will go the distance. Characteristics that are favored by embryologists include same sized cells with little or no fragmentation. The four cleavage stage embryos in the picture at the right are a good example of nice cleavage stage embryos on day 3, when the embryo is expected to have cleaved into at least 8 cells. There is some variability in the cell number that we see on day 3. We expect the best prognosis from embryos that have reached 7-12 cells. If the embryo is only two cells on day 3, this is not a good sign and likely indicates the embryo has ceased to grow. Normal embryos have a fairly strict rate of progression which starts at the time of fertilization. If the time of fertilization is delayed (for example, if rescue ICSI is used), the start time of the embryos progression program is delayed and the embryo may reach the eight cell on day 4, not day 3 of culture since fertilization occurred a day later than expected. But except for delays in fertilization, progression should follow an expected predictable rate. Embryos dont usually speed up to catch up if they are lagging.
