There is significant disagreement about the definition of menopause. Some confusion exists because there are several stages of the natural menopause process. Technically, natural menopause is the transition between perimenopause and postmenopause, the entire process culminating with the ceasing of the menses, generally around age 50 for most women. This natural menopause process itself is usually identified retrospectively, when it's been a year since a last period. It can be viewed as a hormonal shift mirroring puberty.
Natural menopause occurs when the monthly cycle of ovulation comes to an end.
This is because the ovarian supply of follicles and eggs declines sharply as a woman approaches menopause. During this time, called perimenopause, which can last anywhere from five to fifteen years, the brain continues to send out hormones trying to stimulate the development of ovarian follicles, and it is common for a woman's ovaries to respond erratically, so that her hormones fluctuate a great deal from month to month. These fluctuations are responsible for many of the symptoms of perimenopause. Eventually, though, the ovaries are no longer able to develop an egg for ovulation. Ovarian production of estrogen goes into a permanent decline, and progesterone is no longer produced.