Many women also refer to the menstrual period as "AF" which is an abbreviation for "Aunt Flo". Menstruation refers to the monthly bleeding caused by the shedding of the lining of the uterus (endometrium). Even though this bleeding occurs in regular monthly cycles (28-29 days), the actual number of days in most women's cycles can be anywhere between 21 and 35 days. Because menstruation is a function of regular ovulation, it begins in puberty (menarche=first menstruation), and ends with menopause, when ovulation stops. The first day of bleeding is counted as the beginning of each menstrual cycle (cycle day 1 or CD 1) and the menstrual cycle ends on the day before the next menstrual period begins. Menstrual cycles normally range from about 21 to 35 days. Only about one in eight cycles are exactly 28 days.
The menstrual cycle is typically separated into three phases:
Follicular phase is the phase which begins with the first day of menses (Cycle Day 1). During the early part of this phase, the pituitary gland increases release of follicle stimulating hormone (FSH) and luteinizing hormone (LH). FSH stimulates the growth of numerous follicles in the ovary as well as the release of estrogen from the ovary. The estrogen causes thickening of the endometrium. Generally only one of the follicles matures and gets to release the ovum (egg) contained within. LH is secreted in a sudden burst mid-cycle (around Cycle Day 13-14) and stimulates the release of the ovum from the mature follicle in the ovary.
The ovulatory phase lasts usually only one to two days. During this phase, the ovum is released from the mature follicle -- in a process called ovulation -- because of the increased presence of the hormone LH. Ovulation generally occurs in the 16 to 32 hours following follicle exposure to increased LH levels. The follicle ruptures and releases the ovum (egg) into the funnel-shaped end of the fallopian tube. The ovum then makes its way down the tube and into the uterus, hoping to get fertilized along the way.
The luteal phase usually lasts 12-16 days. It is the time following ovulation until the next menstrual period (bleeding). Following ovulation, the follicle that ruptured and released the egg closes and becomes a corpus luteum. The corpus luteum produces increasing levels of progesterone, preparing the endometrium for the implantation of a fertilized egg. If pregnancy does not occur, the corpus luteum breaks down and the levels of progesterone and estrogen decline. This results in the breakdown of the endometrium and menstrual bleeding occurs. If pregnancy does occur, the placenta begins to produce human chorionic gonadotropin (hCG). It is the top and middle endometrial layers that are shed during the menstrual period. The bottom (innermost or deepest) layer remains, and prepares once again for implantation in the next cycle. Menstrual periods and cycles can be disturbed in a number of ways: Click here for more about Menstrual Disturbances.