The menstrual cycle
On average, a woman will experience around 500 menstrual cycles in a lifetime. On average every cycle lasts for about 28 days, although cycles lasting 21 to 35 days are considered normal as well. The first day of the cycle is also the first day of the menstruation or period.
The course of each cycle is controlled by hormones (chemical messengers), which amongst other thing ensure the continuous cycle of fertility.
The endocrine glands produce the hormones, which then are spread throughout the body by the bloodstream. Four of those glands are responsible for the menstrual cycle: The hypothalamus, the pituitary gland, the thyroid and the ovaries. The hormones produced by the pituitary gland and the ovaries, directly influence the menstrual cycle, whilst the hormones of the thyroid regulate the body's energy flow, which enables the other hormones to work normally. The pituitary gland is also responsible for the coordination of the menstrual cycle as well as its stimulation. In between the ovaries, the hypothalamus and the pituitary gland, there is an essential stimulus-response system, which maintains the pace of the cycle.
But also emotions can influence the menstrual cycle, as the hypothalamus is connected with other parts of the brain.
The menstrual cycle consists of three phases:
The pituitary gland releases the follicle-stimulating hormone (FSH) and the luteinizing hormone or lutropin (LH). Via the bloodstream they reach the ovaries and stimulate the development of 10 to 100 follicles. The ovaries then start to produce oestrogen, which effects various parts of the female body.
Normally, only one of the ovaries is stimulated during the menstrual cycle, which means they alternate.
The pituitary gland then releases another hormone: Prolactin. In small concentration, prolactin is essential for ovulating; it is produced throughout the menstrual cycle. However, during breastfeeding, in high concentration, the hormone prevents ovulation and hinders fertility.
Oestrogen, produced in the ovaries affect the hypothalamus, where the production of hormones, affecting the pituitary gland, is stimulated. This is a so-called stimulus-response system, as on their part the hormones produced by the pituitary gland stimulate the development of follicles in the ovary. The follicles produce oestrogen, which affects the hypothalamus and furthermore causes the production of more hormones in the pituitary gland.
Around half way through the cycle, once the oestrogen levels in the blood have reached a high, the pituitary gland releases a dose of luteinizing and follicle-stimulating hormones.. As a consequence one ovum (and sometimes more) is released by a follicle. The other developed follicles, gradually disappear during the 14 remaining days of the menstrual cycle.
Due to the level of oestrogen in the bloodstream, the endometrium (the inner membrane of the uterus) will have swollen and be able to accommodate the fertilized ovum. Furthermore, the hormone stimulates the cells in the uterus to release a more diluted mucus, which is conducive to sperm. This mucus indicates fertility, and can be felt by the woman at the entrance of the vagina.
Around 32 hours after the luteinizing and follicle-stimulating hormones have been released, the follicle releases an ovum, which then moves towards the Fallopian tubes. In the Fallopian tubes, the ovum is nourished and stays available for 12 to 36 hours, in order to be fertilized by a spermatozoon.
3. Luteal Phase
As the ovum absorbs lipids or fats, it starts to turn yellow. Which is where the phase's name comes from: Corpus luteum (yellow body). Continuing to produce oestrogen, as well as starting to produce progesterone. Progesterone has the following effects: It changes the structure ofthe cervical mucus, which turns thick in order to block sperms from entering, then it becomes less; the glands found in the Fallopian tubes and the womb produce a liquid to nourish the ovum and to prepare it for fertilization in the Fallopian tubes, and its implantation in the womb lining. At this point the woman's body temperature will rise by some degrees and will stay risen until the end of the cycle.
If the ovum was not fertilized, the corpus luteum disintegrates, because of the declining levels of progesterone and oestrogen in the bloodstream, during the following 10 days. As the stimulation of the womb lining has ceased, it is shed and the woman's period will start. As the menstrual cycle is coming to an end, the body temperature will return to normal.
If the ovum has been fertilized, some of its cells will start to produce human chronionic gonadotropin (HCG). HCG, is the hormone that is detected by pregnancy tests. Human chorionic gonadotropin keeps the corpus luteum stable, and because of that it can continue to produce progesterone, which keeps the womb lining rich in nutrients. This is vital during the first ten weeks of pregnancy, until the placenta takes over the nourishment of the fertilized ovum.