While there are several methods for categorizing the phases of the menstrual cycle, the one most commonly utilized by Western medicine practitioners identifies four phases:
The menstrual phase is the first part of the follicular phase. The menstrual phase begins on the first day of menstrual flow and lasts until bleeding ends, typically 5 to 7 days. During this time both estrogen and progesterone levels are low, which allows for the shedding of the uterine lining as there is no fertilized egg.
The proliferative phase is the second part of the follicular phase. The proliferative phase begins at the end of the menstrual phase and lasts until ovulation. During this time the pituitary gland secretes follicle stimulating hormone (FSH). FSH stimulates the ovaries to secrete follicles. There is also a rise in estrogen and luteinizing hormone (LH).
The ovulation phase begins with ovulation and lasts approximately 24 hours. During the end of the proliferative phase, rising levels of estrogen stimulate the release of LH. This reaches a peak around day 14 and causes the ovary to release a mature egg; this process is known as ovulation. FSH and LH reach their peak at ovulation. Estrogen reaches its peak at the end of the proliferative phase.
The luteal phase begins after the ovulatory phase, typically around day 15 or 16. During the luteal phase progesterone levels begin to rise. Progesterone reaches its peak around day 21, and is accompanied by a rise in estrogen. Both progesterone and estrogen steadily decline from this point. They reach their low at the end of the luteal phase and then the menstrual phase begins again.
To learn about the Chinese medical view of menstruation, see The Treatment of Perimenopausal Anxiety with Acupuncture and Chinese Medicine.