Why do blastocysts grow on the 7th day?

The blastocyst is a 116 ± 2 hour embryonic developmental stage with a complex cellular structure that consists of approximately 200 cells.

Previously, in ART (assisted reproductive technology) practice, embryos that did not reach the blastocyst stage on day 6 were disposed of in accordance with laboratory standards, but the generally accepted practice of stopping the cultivation of embryos on day 6 was questioned. It was believed that a long delay in the development of embryos is a sign of poor quality, aneuploidy (violation of the set of chromosomes) and, as a result, a low pregnancy rate.

It should be noted that embryos that have reached the blastocyst stage on day 7 may:

keep alive
achieve a high morphological score (have the correct appearance)
be chromosomally and genetically healthy, lead to the birth of healthy babies.
The main factors that can affect the rate of embryo development and blastocyst formation only by the 7th day of cultivation are:

reproductive age of a woman (a decrease in the energy level in mitochondria occurs with an increase in reproductive age). It is important to know that the embryo develops to the stage of implantation only due to the energy that it received from the egg.
FSH dosage
morphology (quality) of oocytes obtained during stimulation.
Bad habits (smoking, alcohol), toxins, resins, varnishes, paints, some drugs affect the decline in egg quality.

The intake of antioxidants and proper nutrition, on the contrary, reduces the level of oxidative stress and can improve the quality of oocytes, affect the proper development of embryos.

percentage of normal spermatozoa
number of previous ART cycles
culture system used, quality of ventilation in the embryology laboratory
in some cases, the genetics of the embryo itself
metabolic and epigenetic processes of the embryo
delay of the first division (crushing)
However, in some cases, the formation of a blastocyst by the 7th day is the result of a combination of several factors.

In addition, 34% of blastocysts at day 7 may be euploid, indicating that slow development is not a direct result of genetic breakdown in all cases. Thus, extended cultivation gives a chance for such a long-awaited pregnancy and the birth of a healthy child.

How did your embryos develop? Were there seven-day ones among them?