Normal cycle :
The uterine cycle begins with menstruation, also called period. These last about 5 days. In the beginning, the uterine lining or endometrium has a maximum thickness. This lining gradually deteriorates. This deterioration is caused by the decline in the production of ovarian hormones, estrogen and progesterone, whose concentrations are minimal.
At the same time, the ovarian cycle begins with the follicular phase. This phase occurs during the first 14 days during which a dominant follicle develops under the action of a hormone produced by the pituitary gland, a gonadotropin called FSH (follicle stimulating hormone). The more the follicle develops, the more estrogen it produces, which induces the endometrium reconstruction. Low estrogen concentrations at the beginning of the cycle exert a negative feedback on the pituitary gland. Thus, the gonadotropin concentrations remain low at the beginning of the cycle. As estrogen concentration increases, feedback becomes positive and the pituitary gland releases more gonadotropins. A peak of LH (luteal hormone) then forms around the 14th day of the cycle. The LH peak triggers ovulation.
The exploded follicle turns into a corpus luteum under the action of LH. The corpus luteum secretes estrogen but also another ovarian hormone, progesterone, whose concentration increases rapidly. Progesterone has an effect on the endometrium which is more vascularized. It also induces negative feedback on the pituitary gland. Thus, the secretion of gonadotropins decreases significantly.
Finally, the corpus luteum degenerates which induces a decrease in the production of ovarian hormones. A new cycle begins again.