Embryology is the study of the beginning stages of human development during the early phases of pregnancy. It is a complex science that you don't need to know too much about, but there are a few concepts to understand that can help make going through treatment a little easier.
Fertilization usually takes place in the Fallopian tube and is the process where a single sperm cell penetrates the egg. Only one sperm cell can penetrate an egg.
As the sperm encounters the cells that surround the egg, special enzymes secreted by the sperm begin to break down those cells. The sperm will then encounter the zona pellucida, which you can compare to the shell of an egg. This is a very important step because a reaction occurs as the sperm penetrates the zona that makes the zona impenetrable to other sperm cells. The maternal chromosomes and paternal chromosomes begin to undergo a series of changes that eventually combines the genetics of the two cells (egg and sperm) into a single, unified genetic code. The two cells are now a single-celled zygote.
Early embryo development
Over the next several days, the zygote begins a series of rapid divisions, into two cells, then four, then eight, etc. Around day three, it becomes a morula; a two-layered spherical structure with twelve to thirty-two cells. Until this point the zygote is still in the Fallopian tube.
Sometimes multiple sperm cells do fertilize an egg. This is usually a result of a defect in either the sperm or the egg. The pregnancy will not continue because of the severity of the impact on the fetus. If this is happening to you, it can't be diagnosed unless directly observed in the lab.
Around day four, the morula enters the uterus and starts to turn into a blastocyst. A small amount of fluid enters the morula from the uterus and begins separating the two layers of cells. The inner layer of cells, called the inner cell mass, will eventually become the fetus. The thin, outer layer of cells, called the outer cell mass, will become the placenta.
The blastocyst will float around in the uterus for approximately two days while it forms. The zona pellucida begins to disintegrate and disappear, allowing the blastocyst to rapidly grow in size. This is called hatching of the blastocyst.
In some women, usually older women, the zona pellucida is thickened, making it difficult for the blastocyst to hatch. If the blastocyst hasn't hatched, it cannot implant in the uterine wall. A thickened zona can only be diagnosed when the embryologist notices it in the lab during IVF.
Six days after fertilization the blastocyst will burrow into the uterine lining, a process known as implantation. The implanted blastocyst will undergo a number of changes as the cells begin to differentiate into not only the developing embryo, but the extra embryonic structures including the placenta, yolk sac, and amniotic cavity. At the end of this week, the pregnancy will start to produce a hormone called human chorionic gonadotropin, or hCG, that will continue to support the pregnancy and the corpus luteum. This will keep the corpus luteum producing abundant amounts of progesterone, which will in turn also support the pregnancy.