By "menstrual cycle" we mean a 28 to 30-day period in which women experience a series of changes within their bodies due to the action of hormones. Every month, these hormones cause the uterine wall to get ready for pregnancy. There are cycles of shorter duration, about 25-26 days.
This cycle includes two main, interrelated processes:
- Ovulation: it is the process by which a mature egg is released from the ovary. Usually, only a single mature egg is released within the so-called "Graafian follicle". This egg then leaves the ovary and moves into the Fallopian tubes. Simultaneously, the uterus is preparing itself to receive the fertilised egg by increasing the size of the endometrium, thereby allowing embryo implantation to occur.
- Menstruation: if the sperm does not reach the egg and fertilisation does not take place, the endometrium will shed, and menstrual bleeding will occur.
The different sections of this article have been assembled into the following table of contents.
This whole process is hormonally controlled by the pituitary gland (a.k.a. hypophysis). Located at the base of the brain, this gland is stimulated by the so-called "gonadotropin-releasing factors" (GnRH). Hence, it is responsible for the release of the two hormones that play a key role within the ovarian cycle: the follicle-stimulating hormone (FSH) and the luteinizing hormone (LH).
Based on hormone levels, the menstrual cycle can be divided into three phases:
During puberty, women have approximately 300,000 microscopic structures within the ovary known as "primordial follicles".
In the pre-ovulatory phase of the menstrual cycle, the level of FSH increases, which means that several follicles start growing and moving into the area of the ovary. The growth of these follicles causes an increase of "estradiol", a hormone which main function is to block the production of FSH by the pituitary gland. In such a way, despite the fact all follicles have started maturing, only one of them will continue its development, therefore preventing the remaining follicles from developing.
This dominant follicle is known as "Graafian follicle", which produces other hormones that have an effect on the uterus. It is responsible for the proliferation of the endometrium that enables the subsequent embryo implantation.
Half-way through the cycle (day 14 or 15), the pituitary gland produces an increase in the levels of LH. This causes the Graafian follicle to burst, thus releasing the mature egg from the ovary into the Fallopian tubes. This process is known as "ovulation".
This egg can be fertilized by sperm present in the fallopian tubes after sexual intercourse.
In the luteal or post-ovulatory phase, the burst follicle develops into a small yellow structure called corpus luteum (Lat. "yellow body"). This structure produces both estrogen and progesterone, two hormones which will have an effect on the endometrium.
Progesterone affects the cervical mucus located in the cervix, making it impenetrable to sperm. It also acts on the lining of the uterus, which becomes thick and spongy, as a result of an increased blood flow and its preparation to receive the fertilized egg.
The corpus luteum keeps on producing these hormones until the 8th week of pregnancy. From that moment onwards, they will be produced by the placenta.
In case the released egg is not fertilised by sperm, the corpus luteum will disappear. Hence, the production of oestrogen and progesterone gradually decreases. This causes the endometrium to shed and be expelled through the vagina during what we know as “menstruation”.
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