In a woman, LH and FSH are both released into the bloodstream in response to the GnRH delivered by the hypothalamus to the pituitary gland. Together with LH and FSH, two other hormones regulate the menstrual cycle. These hormones are oestrogen and progesterone. In addition, oestrogen encourages the development of the female secondary sexual characteristics (Table 1) and most of it is made by the ovaries, but a little oestrogen is also made by the adrenal glands. Progesterone is responsible for thickening the active layer of the uterus or womb, preparing it for possible pregnancy. The main sources of progesterone are the ovaries, but during pregnancy, the production of progesterone is taken care of by the placenta, an organ attached to the uterus which holds and nourishes the growing embryo. LH and FSH are indirectly responsible for the presence of oestrogen and progesterone in the blood.
For the first few days of the menstrual cycle, a certain amount of bleeding occurs. This is known as menstruation. During menstruation (1), progesterone levels are low. Since progesterone helps to maintain the active layer (endometrium) of the uterus, low levels result in the shedding of much of the endometrium, taking tiny blood vessels which have grown over the previous few weeks with it. At this point, the blood contains medium levels of LH and FSH and only a little oestrogen.
Over the next week or so, oestrogen, LH and FSH begin to increase until about halfway through the menstrual cycle, when ovulation (2) occurs. It is during ovulation that an egg is released from the ovary and slowly travels to the uterus. Only one sperm cell is needed to fertilise the released egg cell.
Just before ovulation, LH, FSH and oestrogen levels are high but progesterone is still low. Soon after ovulation, hormone levels decrease rapidly except for progesterone, which gradually reaches its peak level on around the 21st day of the menstrual cycle (3) and the endometrium is therefore at its thickest.
If the released egg is not fertilised during this time, the blood begins to "lose" progesterone. When progesterone is at its lowest level (4), the thick endometrium can no longer be supported. It begins to degenerate, menstruation occurs and the menstrual cycle starts again (1).
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