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The Menstrual Cycle

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Updated May 15, 2014

The menstrual cycle is a complex process which prepares the body for a potential pregnancy each month. A series of hormonal changes beginning in the brain build up a thick lining of tissue in the uterus. An egg is released from the ovary which travels down the fallopian tube in hopes of coming across sperm cells, one of which may fertilize it. If the egg is fertilized, the embryo will then implant into the thickened lining. If fertilization does not occur the egg and the lining are passed through the vagina as menses.

Menarche, or the beginning of menstruation usually occurs in the onset of puberty, around 12 years old. At first, cycles may be irregular or annovulatory, but eventually a regular cycle will emerge. The length of this cycle which is counted from the beginning of one menses to the beginning of the next varies from woman to woman, but averages at 28.

The cycle is initiated when the hypothalamas gland secretes a hormone called Gonadotropin Releasing Hormone (GnRH) which travels to the pituitary gland. Both glands are located in the brain. This hormone is the body’s way of telling the pituitary to start releasing Gonadotropins, the hormones which cause an egg follicle in the ovary to begin growing. The two main Gonadotropins are Follicle Stimulating Hormone (FSH) and Leutenizing Hormone (LH).

The developing egg follicle begins to secrete estrogen, which causes the lining of the uterus (endometrium) to begin to thicken. Once the follicle matures, LH levels increase dramatically, triggering ovulation. Ovulation is the release of a mature egg from an egg follicle within the ovary. This occurs on approximately day 14 of the cycle. After the egg is released, the follicle is then called the corpus luteum and begins to secrete progesterone. Progesterone causes glands and blood vessels within the endometrium to curl and become more complex, preparing it for implantation. If implantation does not occur, estrogen and progesterone levels drop and the lining is shed as menstruation, triggering the beginning of the next cycle.

The menstrual cycle is typically referred to in ovarian and uterine changes. The ovary goes through two phases: the follicular and luteal phases. It is said to be in the follicular phase as the egg follicle is developing. Once ovulation occurs, the ovary enters the luteal phase where it remains until the next cycle. Uterine phases are a little more complex. During menstruation (usually day 1-6), the uterus is in the menstrual phase. Once menses has finished, and as the endometrial lining begins to build up again, the uterus is in the proliferative phase. The secretory phase occurs after ovulation occurs, once the glands and blood vessels in the endometrium prepare the lining for implantation. If implantation does not occur, progesterone and estrogen drop, causing the uterus to enter the ischemic phase and the lining to disintegrate.

This may seem complicated, but if any of the steps do not happen, your cycle will be affected. It is helpful to understand the basic steps that your body is supposed to go through each month so that you can have a more meaningful conversation with your doctor if you think something is abnormal. It may be helpful to keep track of your cycle on a calendar so you can give your doctor concrete information about your concerns.

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