What Is a Menstrual Cycle?

A good knowledge about the menstrual cycle is important to increase your chances of getting pregnant and to have an idea of what’s normal and not within your reproductive system.

What is menstrual cycle?

The Menstrual cycle is the sequence of changes a woman’s body goes through to prepare for a pregnancy. In women, the fertile period starts at the menarche (first menstrual period) and ends with the menopause. About once a month, the uterus grows a new lining (endometrium) to get ready for a fertilised egg. When there is no fertilised egg to start a pregnancy, the uterus sheds its lining. This is the monthly menstrual bleeding (also called menstrual period). The menstrual cycle is from Day 1 of bleeding to Day 1 of the next time of bleeding.

This period is divided in cycles of 28 to 35 days in length separated by menstruation. Although the average cycle is 28 days, it is perfectly normal to have a cycle that is as short as 21 days or as long as 35 days. For a teen, a normal cycle can last up to 45 days.

Girls usually start having menstrual periods between the ages of 11 and 14. Women usually start to have fewer periods between ages 39 and 51. Women in their 40’s and teens may have cycles that are longer or change a lot. If you are a teen, your cycles should even out with time. If you are nearing menopause, your cycles will probably get longer and then will stop.

menstural cycle

What are the phases of the menstrual cycle?

Menstrual Phase
In the menstrual cycle, the uterus inside a woman’s body builds up a brand new lining of blood and tissue. It will help accommodate a developing baby if the egg was fertilised. When this lining, called the endometrium, has no ovum that will attach in it, it sloughs off within the uterus and travels down the cervix and the vagina causing your menstrual period. This is the phase where it signifies that a woman has not become pregnant. Regular menstruation normally lasts for 3 to 5 days.

Follicular Phase

  • (Begins on the first day of menstrual bleeding) – 13 or 14 days

The lining of the uterus (endometrium) is thick with fluids and nutrients intended to nourish an embryo. If no egg has been fertilised, estrogen and progesterone levels are low. As a result, the top layers of the endometrium are shed, and menstrual bleeding occurs.

The pituitary gland then slightly increases the production of follicle-stimulating hormone. This hormone then stimulates the growth of 3 to 30 follicles. Each follicle contains an egg. Later in the phase, as the level of this hormone decreases, only one of these follicles (called the dominant follicle) continues to grow. It soon begins to produce estrogen, and the other stimulated follicles begin to break down.

Of the three phases, this phase varies the most in length. It tends to become shorter near menopause. This phase ends when the level of luteinising hormone increases dramatically (surges). The surge results in release of the egg (ovulation).

Ovulatory Phase

  • (begins when the level of luteinising hormone surges) – 16 to 32 hours

This phase is important to determine if you want to get pregnant. In this phase, luteinising hormone stimulates the dominant follicle to bulge from the surface of the ovary and finally rupture, releasing the egg. The level of follicle-stimulating hormone increases to a lesser degree. The egg can be fertilised for only up to about 12 hours after its release. Fertilisation is more likely when sperm are already present in the reproductive tract before the egg is released.

Around the time of ovulation, some women feel a dull pain on one side of the lower abdomen. This pain is known as mittelschmerz (literally, middle pain). The pain may last for a few minutes to a few hours. The pain is felt on the same side as the ovary that released the egg, but the precise cause of the pain is unknown. The pain may precede or follow the rupture of the follicle and may not occur in all cycles. Egg release does not alternate between the two ovaries and appears to be random. If one ovary is removed, the remaining ovary releases an egg every month.

You can also use other ways to pinpoint your time of ovulation such as:

  • Checking your cervical mucus
  • Taking your basal body temperature
  • Using an ovulation calendar
  • Keeping an eye on your luteal phase

Luteal Phase

  • (begins after ovulation) – 14 days unless fertilization occurs

In this phase, the ruptured follicle closes after releasing the egg and forms a structure called a corpus luteum, which produces increasing quantities of progesterone. The corpus luteum prepares the uterus if fertilisation occurs. The progesterone produced by the corpus luteum causes the endometrium to thicken, filling with fluids and nutrients to nourish a potential fetus.

It also causes the mucus in the cervix to thicken, so that sperm or bacteria are less likely to enter the uterus. Progesterone also causes body temperature to increase slightly during the luteal phase and remain elevated until a menstrual period begins. This increase in temperature can be used to estimate whether ovulation has occurred. During most of the luteal phase, the estrogen level is high. Estrogen also stimulates the endometrium to thicken. This phase ends just before a menstrual period.

What are the hormones involved?

The menstrual cycle is part of an incorporated system which includes the hypothalamus, the pituitary gland, the ovary and the uterus. The biological clock which is responsible for the rhythmicity of the cycles consists in the pulsatile release of a hypothalamic decapeptide: the Gonadotropin Releasing Hormone (GnRH). The pulsatile secretion of GnRH depends on the ovarian events through the feed-back effect that the sexual steroids exert on the hypothalamus and the pituitary. This modulated secretion of GnRH controls the release and synthesis of the polypeptidic pituitary gonadotropins: Follicle Stimulating Hormone (FSH) and Luteinising Hormone (LH).

FSH allows recruitment and growth of the ovarian follicles as well as the selection of the dominant follicle whereas LH induces follicular rupture and sustains the corpus luteum. The follicles and the corpus luteum produce oestradiol and progesterone where their secretion is dependent to the gonadotropin. Bound to transport proteins, these steroids are carried in the blood stream and regulate the secretion of GnRH, FSH and LH and induce proliferation and differentiation of the uterine endometrium in order to guarantee implantation of the embryo if fertilisation has taken place.

Gonadotropin-Releasing Hormone (GnRH)

– Produced by the hypothalamus (a part of the brain) and stimulates the release of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) from the pituitary gland.

Follicle Stimulating Hormone (FSH)

– Produced by the pituitary gland and stimulates the growth of follicles in the ovaries, (a follicle is a balloon shaped structure which is filled with fluid and contains an egg, follicles are found in the ovaries) to ripen several eggs and secretes estrogen.

Lutenising Hormone (LH)

– Produced by the pituitary gland. It helps in the development of the follicles, and works with the FSH that triggers ovulation and stimulates production of other hormones necessary for the post ovulatory stage of the menstrual cycle – estrogen and progesterone.


– Hormone secreted by the ovarian follicles and the corpus luteum that stimulates thickening of the uterine wall, maturation of an egg, and development of female sex characteristics. Also provides feedback to inhibit FSH secretion and increase LH secretion.


– Hormone secreted by the corpus luteum that stimulates thickening of the uterine wall and formation of mammary ducts.

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