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The Menstrual Cycle: How it Works

Menarche is a major landmark of puberty and marks the beginning of menstruation. This happens approximately between the ages of 9 to 16 years of age. During menstruation, which typically lasts between 2 and 7 days, the lining of the uterus is shed and passes through the cervix to the vagina and out of the body as menstrual blood.

The total amount of blood that flows during an average period seems to be so much more than it actually is; it’s usually only about 4 teaspoons. But the range is between 1 and 4 tablespoons. The consistency of the blood flow is usually thick and bright red during heaviest flow. It is not unusual to see clots during this time. The thickest and most viscous flow happens as the richest part of the lining literally un-knits from the uterine wall. Gradually, as the flow progresses, it thins and becomes browner in color.

The menstrual cycle begins on the first day of the period and ends on the first day of the following period. The cycle lasts an average of 28 days but can range from 21 to 45 days.

The follicular phase

The follicular phase is the first phase of the cycle. It lasts about 10 to 14 days. The uterine lining rebuilds in preparation for ovulation and hormones stimulate follicle development during this phase. The drop of estrogen levels triggers an increase of follicle-stimulating hormone (FSH) that promotes the growth of a group of follicles within the ovaries. One follicle will continue to develop to a mature egg. Hormones cause the mature follicle to burst and release the egg into the fallopian tube. This is the beginning of ovulation, which marks the end of the follicular phase.

Luteal phase

The cycle enters the luteal phase with the ovulation of an egg from one of the ovaries. A surge of luteinizing hormone initiates ovulation. Estrogen and progesterone levels also rise during the luteal phase causing the endometrium (the lining of the uterus) to thicken in preparation for an embryo (a fertilized egg).

If a woman becomes pregnant, the embryo enters the uterus and implants itself in the endometrium within the first few days after ovulation. Human chorionic gonadotropin (HCG), produced by the placenta, interrupts the menstrual cycle by constantly stimulating the burst follicle (corpus luteum) to produce estrogen and progesterone. The high levels of estrogen and progesterone prevent the endometrium from shedding. If conception does not occur, estrogen and progesterone levels decrease, signaling the uterus to begin shedding the endometrium. The luteal phase lasts until the beginning of the next period.

Hormonal Consequences

Fluctuating hormone levels during the menstrual cycle also cause a number of other bodily changes. There are increases and decreases in temperature and vaginal discharge, as well as abdominal twinges that can be associated with ovulation or the beginning of menstruation. For some women these symptoms are minor. For others, they wreak real havoc with day-to-day activities. In all cases, understanding your premenstrual symptoms and the ebb and flow of your overall menstrual cycle can help you navigate and deal with your individual rhythms.

The follicular phase

The follicular phase is the first phase of the cycle. It lasts about 10 to 14 days.

The luteal phase

The cycle enters the luteal phase with the ovulation. It lasts until the next period.

Periods

If conception does not occur, estrogen and progesterone levels decrease and periods start.