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CSEC Biology: The Menstrual Cycle

Between the first menstrual cycle (menarche) a female has during puberty and menopause, a woman's body undergoes a monthly hormone-driven set of changes to prepare it for a potential pregnancy. This monthly cycle of changes is known as the menstrual cycle. (Please note that the menstrual cycle is not the same as menstruation. The difference is that menstruation is the elimination of the thickened endometrial layer while the menstrual cycle is the entire monthly set of changes including menstruation).

The menstrual cycle is split up into four phases:

  1. Menstruation

  2. The Follicular Phase

  3. Ovulation

  4. The Luteal Phase


Menstruation occurs when the egg from the last cycle is not fertilized. This is when a female is said to 'get their period.'

Since a pregnancy has not occurred, the levels of estrogen and progesterone (hormones) fall. As a result, the thickened inner layer of the uterus (the endometrium) is no longer needed, and is broken down. The layer is shed as menstrual flows, which usually comprise blood, mucus and cells from the endometrium (endometrial cells).

You could think of this as the egg being a girl all prepared for a boy to take her to prom (fertilize the egg). However, on prom night, the boy (a sperm cell) completely bails and does not show up to fertilize the egg, forcing her to have to get rid of all the painstaking preparations. Yes, think of it like that, but every month for an average 40 years.

This phase will last for 3-7 days.

The Follicular Phase

The follicular phase actually also starts on the same day as the first day of menstruation.

At this phase, the hypothalamus (a small region of the brain responsible for regulating body temperature and releasing hormones) tells the pituitary gland (a 'master gland' in the brain responsible for controlling the activity of most of the glands in your body) to begin releasing Follicle Stimulating Hormone (FSH). This hormone tells the ovaries to produce between 5 and 20 small sacs known as follicles. Each follicle contains an immature ovum, or egg. Often, only one follicle keeps growing to maturity and travels to the ovarian surface while the remaining follicles gradually fade and are absorbed back into the body.

The developing follicle causes a rise in the level of estrogen, which causes the lining of the uterus (endometrium) to thicken in preparation for a potential pregnancy. The hypothalamus in the brain recognizes these rising levels and releases a chemical called gonadotrophin-releasing hormone (GnRH). This hormone prompts the pituitary gland to produce raised levels of luteinising hormone (LH) and FSH.

This lasts for an average of 16 days, however, it can range between 11-27 days based on a woman's particular cycle.


The release of luteinising hormone in the follicular phase triggers ovulation. Ovulation is the release of a mature egg from the ovary. The mature egg is extracted from the follicle (cyst) and is transported from the ovary to the fallopian tube. If fertilization does not occur during this phase, the egg continues to the uterus and dissolves within 6 to 24 hours.

This phase lasts for about a day.

The Luteal Phase

In ovulation, the egg is burst out of its follicle, but the ruptured follicle stays on the surface of the ovary. For the next two weeks or so, the follicle transforms into a structure known as the corpus luteum. This structure starts releasing progesterone, along with small amounts of estrogen. This combination of hormones maintains the thickened lining of the uterus, waiting for a fertilized egg to implant itself in the endometrium.

If a fertilised egg implants in the lining of the uterus, it produces the hormones that are necessary to maintain the corpus luteum. This includes human chorionic gonadotrophin (HCG),( the hormone that is detected in a urine test for pregnancy). The corpus luteum keeps producing the raised levels of progesterone that are needed to maintain the thickened lining of the uterus.

If pregnancy does not occur, the corpus luteum withers and dies, usually around day 22 in a 28-day cycle. The drop in progesterone levels causes the lining of the uterus to fall away, starting the cycle all over again.

The luteal phase is a sort of 'make or break phase' based on what happens in ovulation. If the egg is fertilized, it implants itself in the uterine wall (endometrium) and it makes the hormones necessary to maintain the corpus luteum and the uterine wall. However, if the egg is not fertilized, the hormones are not released, so the thickened uterine lining breaks down (menstruation).

Diagram Showing the Menstrual Cycle

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18 de jan.

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