Menstrual Cycle

Last Updated: November 30, 2017

Definition - What does Menstrual Cycle mean?

The menstrual cycle is a cycle of changes which the female body goes through to make pregnancy possible. During the menstrual cycle, eggs are brought to maturation and made available for fertilization by the ovaries. Along with the release of eggs, the uterine lining is prepared to accept a possible fertilized egg and begin a pregnancy. Menstruation occurs when the uterine lining is shed if no pregnancy occurs.

The menstrual cycle can range from 21-35 days in healthy women. Cycles longer or shorter than that often indicate a hormonal problem that may negatively impact fertility.

FertilitySmarts explains Menstrual Cycle

The three phases of the menstrual cycle are:

  1. The follicular phase starts on the first day of menstrual bleeding, where the uterine lining from the is shed and expelled through the vagina. The main event of this phase is the development of follicles in the ovaries. Follicle-stimulating hormone stimulates the growth of 3-30 follicles, each containing an egg. Only one follicle will become dominant and the other stimulated follicles break down. This phases lasts around 13-14 days.
  2. The ovulatory phase starts when the level of lutenizing hormone begins to rise, and within 12-14 hours, triggers one (or more) dominant follicles release a mature egg which is ready for fertilization. The egg can only be fertilized for around 12 hours after its release. At the time of ovulation, basal body temperature begins to rise.
  3. The luteal phase starts after ovulation and concludes just prior to menstruation. The ruptured follicle that housed the egg becomes a structure known as the corpus luteum that starts to produce progesterone. This progesterone causes the uterine lining to build up in preparation to accept a potentially fertilized egg. Progesterone can cause a slightly elevated body temperature, which can be used to estimate if ovulation occurred.

If the egg is not fertilized or does not properly implant, the corpus luteum will degenerate after 14 days. This causes levels of estrogen and progesterone to decrease, and a new menstrual cycle begins.

If an embryo is implanted, the hormone human chorionic gonadotropin (hCG) is produced. hCG maintains the corpus luteum (which continues to produce progesterone that maintains the uterine lining) until the fetus can produce hormones.

If any one of these three stages of the menstrual cycle are completed properly, the chances of pregnancy decrease.

Problems with one or more phases of the menstrual cycle may be signaled by periods which are longer than 35 days (in which case ovulation is likely not occurring regularly) or shorter than 21 days (in which menstruation is likely occurring without ovulation occurring at all).

Menstruation may be proceeded by cramps in the pelvis or lower back as the body begins to squeeze blood vessels shut in preparation for shedding the uterine lining. Some cramping is normal, but severe or excessive cramps or menstrual bleeding may also signal problems with the hormones, ovaries, or uterus which may impair fertility if not addressed.

Problems causing abnormal menstruation and low fertility may be treated with hormones to encourage ovulation, and the buildup of uterine lining.

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