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The Phases of Your Menstrual Cycle

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Updated July 19, 2009

Your Menstrual Cycle in Phases

The menstrual cycle is divided into two phases--the follicular or proliferative phase; and the luteal or ovulatory phase. The follicular phase includes the time when menstruation occurs and is followed by proliferation or the growth and thickening of the endometrium. This phase typically lasts from 10-14 days, starting with the first day of menstruation.

Estrogen and progesterone levels are at their lowest during menstruation. When bleeding stops, the proliferative phase begins causing the endometrium to grow and thicken in preparation for pregnancy. During the next (approximately) two weeks, FSH levels rise causing maturation of several ovarian follicles and the size of the eggs triple.

FSH also signals the ovaries to begin producing estrogen which stimulates LH levels until around day 14 of your cycle when one of the follicles bursts, and the largest egg is released into one of the fallopian tubes.

This phase is followed by the premenstrual phase, known as the luteal phase. This premenstrual period lasts approximately 14 days. After ovulation, LH causes the corpus leuteum to develop from the ruptured follicle. The corpus leuteum produces progesterone.

Together estrogen and progesterone stimulate the endometrium to prepare a thick blanket of blood vessels that will support a fertilized egg should pregnancy occur. When pregnancy occurs this blanket of blood vessels becomes the placenta which surrounds the fetus until birth.

When pregnancy does not occur, the corpus leuteum deteriorates and becomes the corpus albicans. Once this occurs, progesterone and estrogen levels decline, and the endometrial lining is shed during menstruation.

Did you know?

Menstruation can vary greatly from woman to woman and from month to month and still be normal. Generally, the length of your menstrual cycle can fluctuate from 3 weeks to 5 weeks, without alarm.

When counting the days in your cycle, always count the first day of your period as day one. The average period lasts about 6 days, although some women may experience slightly shorter or longer periods and be perfectly normal.

Variations in the amount of menstrual flow and the timing of menstruation are quite normal in young women during the first few years following the onset of menstruation. Periods may be irregular or very light. The use of oral contraceptives can often cause fluctuations in menstruation which include either light periods or spotting/bleeding between periods.

Many times, young women are frightened when they discover dark clumps of tissue in their menstruation. Most often, this is a part of the endometrium (uterine lining) shedding and is nothing to be concerned about.

The average age of the onset of menstruation is about 12 or 13, however it may begin as young as 8 for some girls or not until 14 or 15 for others. If your period has not started by the time you are 16, see your physician to assess whether there may be an underlying condition.

Menstruation Resource Center

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