How a pregnancy starts
This fact sheet describes the different stages of the process through which a pregnancy begins. It aims to facilitate the comprehension of the modules of this seminar.
Preparation of the uterus: “the mucus membrane lining the walls of the uterus, called endometrium, undergoes a transformation through cell multiplication, blood vessel development, oedema, and glycogen secretion, ending up in the secretory endometrium, which is thick, receptive, highly vascularized, ready to welcome an embryo. This is called the ‘decidualization’ of the endometrium”.
Fertilization: “the ovum that has been released by the ‘Graafian follicle’ in the ovary, moves into the corresponding Fallopian tube. Here, it can be reached by spermatozoa, of which only one will be admitted into the oocyte in order to fertilize it. As soon as the outer membrane of the oocyte (zona pellucida) is penetrated by a sperm cell, an activation chain is set off in the oocyte: the zona pellucid is rendered impenetrable to other sperm cells, the metabolism of the oocyte’s cytoplasm is activated, the second mitotic division of the oocyte is completed and two pronuclei are formed (from the oocyte’s nucleus and the sperm’s chromatin). The zygote is thereby formed, a small single-celled being of intense activity whose two pronuclei unite their genetic material (from the mother and the father) to form a new genome, different from the father’s and the mother’s, which is unique and cannot be duplicated and will characterize the new being for the entire duration of his or her life. This little zygote is the starting point of a new life, the first form of the human being that immediately starts operating its development. This development will lead him to become successively an embryo, a foetus, a new-born baby, a child, an adolescent, an adult and an elderly person, before ending his life cycle in natural death”.
Implantation and nidation of the embryo: “the fertilized egg, sliding on the secretions produced by the Fallopian tube and led to the uterus by the contractions of the tube, arrives in the uterine cavity five days after fertilization, at the stage of ‘morula’. The polarization of its cells and the apparition of the blastocoel cavity mark its passage to the state of blastocyst, with an inner part (the inner cell mass, which will remain inside the egg) and an outer part (the trophoblast, which is attracted to the uterine mucus membrane, adheres to it and immediately starts proliferating in the thickness of this mucus, where it will develop the placenta). It is the beginning of nidation. The implantation of the blastocyst starts on the sixth or seventh day after fertilization, and ends on the ninth day”.
Menstruation: “if fertilization does not take place, the ovum is eliminated. The endometrium then undergoes an ischemic necrosis (blood flow stops and cells die) and is evacuated, marking the end of the menstrual cycle”.