Oocytes are produced in the two ovaries located next to the fallopian tubes.
Every woman is born with more than 3.000.000 immature oocytes (follicles). During puberty, they are reduced to 400.000; of these, only 400 will mature and become capable of fertilisation. Usually, one mature oocyte (egg) is produced during each cycle. Thus, 400 eggs are generally expected to be produced during a woman’s fertile years.
Normally, it takes about 2 weeks for an oocyte to mature and to be released. At the beginning of the cycle, the hypothalamus activates the pituitary gland in the brain to start releasing a hormone called FSH (follicle-stimulating hormone). FSH then activates the ovaries, where one or more follicles start maturing. During this period, follicles produce estrogens, which down-regulate FSH (FSH levels are reduced). As a result, only one follicle is allowed to mature.
At the same time, the continuous increase in estrogen levels triggers the production of another hormone, called the luteinizing hormone (LH), by the pituitary gland. In a normal cycle of 28 days, the sudden surge of LH (around the 12th day) helps the follicle mature and release the egg (ovulation). This happens 36 hours after the LH surge. Following its release, the egg moves to the fallopian tubes where fertilisation takes place. After ovulation, the empty follicle forms a cyst called ‘corpus luteum’; the latter is very important because it produces and releases hormones, such as progesterone, which help the endometrium (the lining of the uterus), mature and prepare for embryo implantation.
If the egg is not fertilised, progesterone levels are reduced. As a result, the endometrium cannot be sustained and 14 days later, the woman gets her period.