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The menstrual cycle is a crucial process in the female reproductive system, enabling the release of eggs for potential fertilization. This cycle involves the hypothalamus, pituitary gland, and ovaries, typically spanning around 28 days, though this duration can vary significantly among individuals.
The menstrual cycle is generally divided into two main phases:
- the follicular phase. During this phase, hormonal activities, including the release of LH, FSH, and estrogen, promote the development of the follicle, which houses and supports the egg. This phase also prepares the uterine lining for a potential fertilized egg. Midway through the cycle, a surge in LH and FSH triggers ovulation, the release of the egg, marking the ideal time for fertilization.
- the luteal phase. In this phase, the remnants of the follicle, known as the corpus luteum, continue to produce estrogen and progesterone to sustain the uterine lining. If fertilization occurs, this phase supports early pregnancy. If not, the corpus luteum deteriorates, leading to the shedding of the uterine lining, the start of menstruation, and the beginning of a new cycle.
Explore the following pages for an in-depth analysis of the events that unfold during the menstrual cycle.
Menses (approx. Day 0 to 5)
The menstrual cycle typically starts with menses, characterized by uterine lining shedding and bleeding, lasting between 5 to 7 days (varies among individuals). Key events during menses include:
- Estrogen and progesterone levels from the previous cycle decline to their lowest points.
- GnRH pulses from the hypothalamus trigger small releases of LH and FSH from the pituitary gland. These hormones stimulate the growth of multiple follicles (each housing an egg) in the ovaries.
- The follicles produce estrogen. The dominant follicle, which develops faster, secretes more estrogen, inhibiting the growth of other follicles. This follicle forms granulosa cells and a fluid-filled cavity called the antrum, continuing to release estrogen.
Follicular Phase (approx. Day 6 to 14)
Follicular Phase: Part I (approx. Day 6 to 10)
- The primary follicle grows larger as development progresses. The antrum expands, increasing internal pressure within the follicle. Estrogen secretion from the follicle accelerates.
- Blood estrogen levels rise, eventually signaling the brain that the egg is prepared for release.
- Elevated estrogen levels promote the growth of uterine tissue and blood vessels, thickening the lining in anticipation of a fertilized egg.
- LH and FSH levels remain consistently low, aiding the follicle's development.
Follicular Phase: Part II (approx. Day 11 to 14)
- The primary follicle nears maturity, growing larger and preparing for release. The antrum expands further, with internal pressure reaching its peak. Estrogen secretion continues to increase rapidly.
- Estrogen levels peak, indicating the egg is ready for release.
- LH and FSH levels remain steady and low, supporting the follicle's final development.
- The uterine lining thickens further under estrogen's influence, while the cervical mucus becomes thin and watery.
Ovulation (approx. Day 15)
The elevated estrogen levels have informed the brain that ovulation is imminent.
- The hypothalamus releases a significant amount of GnRH, prompting the anterior pituitary to produce large quantities of LH and FSH (mid-cycle LH/FSH peak). This surge lasts for about 24 hours.
- LH and FSH activate enzymes within the follicle. These enzymatic reactions, combined with the pressure from the antrum, cause the egg to be released. The egg moves into the Fallopian tube, where fertilization may occur. The egg remains viable for approximately 12 to 24 hours post-ovulation. The remaining follicle, now called the corpus luteum, stays in the ovary and secretes estrogen and progesterone.
- The uterine lining reaches its maximum thickness, preparing to receive a fertilized egg.
- The cervical mucus becomes extremely thin.
This period represents the ideal window for fertilization.
In non-primate mammals, fully-developed eggs remain in the ovaries during specific times of the year, awaiting a brain signal to ovulate. The brain evaluates factors like temperature and daylight to determine the right time. During ovulation, the female enters estrus or heat, mating with one or more males.
In primates, the ovary informs the brain when it is ready to ovulate. Primates can ovulate monthly throughout the year.
Luteal Phase (approx. Day 16 to 28)
The corpus luteum begins to break down.
The egg is propelled forward by rhythmic, wave-like movements of the finger-like structures lining the Fallopian tube walls.
LH and FSH levels return to their baseline, steady state.
Estrogen levels initially drop after the LH/FSH surge but recover as the corpus luteum continues to produce estrogen and progesterone.
The uterine lining stays thickened, prepared to support a fertilized egg or developing embryo.
The corpus luteum gradually shrinks and deteriorates. It is programmed to degenerate within 14 days unless stimulated by human chorionic gonadotropin (hCG), a hormone produced by an early embryo.
- If fertilization takes place and the embryo successfully implants in the uterine lining, hCG prevents the corpus luteum from degenerating, allowing it to continue producing estrogen and progesterone throughout the pregnancy.
- If fertilization does not occur, the corpus luteum deteriorates by the end of the luteal phase. The unfertilized egg also disintegrates and is expelled during menstruation. As estrogen and progesterone levels drop, menstruation begins, shedding the uterine lining and initiating a new cycle.
