On cue from follicle-stimulating hormone (FSH), which triggers an egg to mature, estrogen starts the endometrial buildup and controls the first part of the cycle. Normally estrogen production gradually builds to a peak just before ovulation, then drops for the remainder of the cycle, dropping again at the end.

On the other hand, the ovaries dramatically increase their output of progesterone at the time of ovulation, prompted by luteinizing hormone (LH). The level of progesterone rises rapidly to a peak in three or four days, surpassing the level of estrogen, and remains elevated in order to develop and maintain the endometrium in the event of conception. Progesterone dominates and controls the cycle during the latter half.

If conception does not occur, within ten or twelve days the levels of both progesterone and estrogen drop quickly, triggering menses and the cycle starts over again. The dramatic drop of progesterone is the trigger that causes the body to shed the endometrium. When progesterone levels are low your menstrual period becomes irregular or infrequent, and the lining of the uterus stays thick and full of blood. Then, if that lining stays thickened month after month, it can be unhealthy. This can increase the chances of developing hyperplasia, or an overgrowth of cells in the uterine lining, which can lead to endometrial cancer. In addition, it had been discovered that the presence of sufficient progesterone prior to ovulation (that is, for at least a week prior to its normal surge) prevents the release of an egg by either ovary.

At menopause the body may reduce its production of estrogen but almost completely halts the production of progesterone. The amount of estrogen in the body drops below what is necessary to start another menstrual cycle – so no cycle can start. However, estrogen is still very much present. The level of progesterone, on the other hand, drops to near zero in the ovaries. For all practical purposes, the female body becomes deficient in progesterone ESTROGEN DOMINANCE. This then, is the condition of a woman’s body prior to menopause (peri-menopause) when menstrual cycles fluctuate (secondary amenorrhea) due to insufficient progesterone. After menopause – lower levels of estrogen accompanied by a virtual absence of progesterone causes the cessation of menses.

Diet and lifestyle play an important role in setting the stage for estrogen dominance in women of reproductive age resulting in premenstrual syndrome. Young girls who are exposed to xenoestrogens in their environment can experience premature female development.