what causes the decreases of the oestrogen levels and on which days of the cycle it occured?
The decrease in estrogen levels during the menstrual cycle is primarily caused by hormonal changes that occur in the ovaries. Estrogen is mainly produced by the follicles in the ovaries, which are structures that contain eggs.
During the first half of the menstrual cycle, known as the follicular phase, estrogen levels gradually increase. This increase is mainly driven by the growth of the dominant follicle, which accumulates estrogen as it matures.
Around the midpoint of the menstrual cycle, typically around day 14 in a 28-day cycle, there is a surge in luteinizing hormone (LH) that triggers ovulation, the release of the mature egg from the dominant follicle. After ovulation, the remaining cells in the follicle undergo transformations and become the corpus luteum.
Following ovulation, during the second half of the cycle known as the luteal phase, the corpus luteum begins producing progesterone, which inhibits further estrogen production. As a result, estrogen levels start to decrease in the later stages of the cycle.
If pregnancy doesn't occur, the corpus luteum will regress, leading to a decrease in both estrogen and progesterone levels. This hormonal decline triggers the shedding of the uterine lining, resulting in menstruation and the start of a new cycle.
So, to answer your question, the decrease in estrogen levels typically occurs during the luteal phase of the menstrual cycle after ovulation has taken place.