The menstrual cycle
The female cycle is driven by a complex interaction of hormones that need to be secreted at the correct level for fertility. The cycle is divided into two phases: the follicular phase, which lasts from day 1 of your period until ovulation; and the luteal phase, which lasts from ovulation until the start of your next period.
the follicular phase On day 1 of the menstrual cycle, the hypothalamus (often referred to as the control centre in the brain) secretes gonadotrophin-releasing hormone (GnRH). This tells the pituitary gland, situated deep inside the brain, to produce follicle-stimulating hormone (FSH). Over the next couple of weeks, the levels of FSH in the bloodstream rise and enable sac-like follicles in the ovaries to grow. Each follicle contains an egg and although around 20 eggs start to ripen each month, only one (or occasionally two) will become fully mature. The others
At ovulation, the egg follicle bursts and the mature egg inside it is released will gradually shrivel up and disappear. Each egg is surrounded by granulosa cells, which feed it and also produce oestrogen. Oestrogen has many roles during the follicular phase:
Rising levels of oestrogen tell the pituitary gland to reduce the production of FSH so that, usually, only one egg is released at ovulation.
Oestrogen starts to thicken the lining of the uterus (the endometrium) in preparation for implantation of an embryo, should fertilization of the egg occur. Oestrogen opens up the cervix and thins the cervical secretions, assisting the passage of sperm.
Oestrogen tells the hypothalamus that the follicle is mature; the hypothalamus then sends a message back to the pituitary gland to produce a short burst, or pulse, of luteinising hormone (LH). This enables the follicle to burst, usually 24 to 36 hours later, and the fully mature egg inside it to be released. This is known as ovulation.
Ovulation takes place from one ovary only, although there is no evidence to indicate that it occurs on alternate sides from one month to the next.
The follicular phase can be quite varied in length.
It typically lasts around 14 days but it will be shorter or longer depending on the length of your cycle and on whether or not your cycles are irregular.
the luteal phase Post ovulation, the ruptured follicle continues to receive pulses of LH as it turns into a small cyst-like swelling called the corpus luteum that starts to produce progesterone, which has several effects:
It thickens the endometrium.
It produces the nutrients that maintain a pregnancy until the placenta can take over.
It switches off secretion of FSH and LH.
J It closes the cervix and thickens the cervical secretions, preventing passage of sperm.
It raises body temperature by approximately 0.2Â°C, thus preparing the uterus for a fertilized egg.