We have the first day when the fertility specialist receives eggs by transvaginal puncture. We fertilize these eggs in a test tube, sometimes we just add a sperm suspension to them, so that fertilization happens by itself. Sometimes, if we have a severe male factor of infertility, we take single spermatozoa, which in this case are very few, and inject them with a special microinstrument into the oocyte in order to make their task easier. The next day we observe fertilization, i.e. if there was a fusion of a sperm and an egg, the next day we will see a fertilized egg, a zygote. Until the third day of development, splitting into many cells of ever decreasing size occurs. All this time, the embryo develops due to the substances accumulated in the egg. On the third day, the embryonic genome is activated, the embryo’s own genes are switched on, and it begins to synthesize its building blocks.
And at this stage, the development of embryos with the most severe genetic abnormalities stops. For example, in Down syndrome, there are three 21st chromosomes. If we have three first chromosomes or one first chromosome, then such an embryo will not even live up to the moment of implantation, its development will stop much earlier. The larger the genetic disorder, the earlier the development of the embryo stops. If this happens not in a test tube, but in natural conditions, then we simply do not see it. Pregnancy does not occur for several months in a row. The embryo stopped on the third day, the tiny speck disappeared.
Further, if the embryo successfully passes this stage, it develops into a blastocyst. This is an embryo that is already slightly larger, about 0.15 millimeters. It consists of a compact cell mass, which will become an embryo, and a cell membrane, which will be needed in order to be implanted into the wall of the uterus. All this time, the embryo is inside its protective shell, the so-called zona pellucida. After we plant it, it continues to grow, increase in size, and at some point it breaks this shiny shell, leaves it. And at this moment he becomes able to implant into the wall of the uterus. This occurs on the eighth or ninth day of development. Around the same period, the implantation window passes through the woman’s body, that is, the moment when she is ready for embryo implantation. If the embryo is healthy, the endometrium of the uterus is ready to receive the embryo, then implantation occurs and pregnancy occurs
