How the menstrual cycle works, all about the phases and its duration

The menstrual cycle is the mechanism by which a woman’s body prepares to welcome a possible pregnancy during childbearing age.

The menstrual cycle or ovarian cycle indicates the entire period between  one menstrual period and the next . We present below: phases of the menstrual cycle; length of the menstrual cycle.

 Often, erroneously, the days in which there is blood flow are called a period (we say “I have a period”), while the right term to refer to those days would be menstrual flow or menstruation

The first period, called menarche , appears for the first time between the ages of 10 and 16. Around the age of 50 (but this is highly variable) the cycle disappears and we enter the menopause phase .

Menstrual cycles are usually irregular after menarche and can take up to 5 years to regularize.

The menarche (which as we said is the first menstrual period) occurs about 2 or 3 years after the start of breast growth.

A predetermined fertility

A woman is already born with a limited supply of reproductive cells. During fetal life, around the fifth month of pregnancy, a girl has a total of about 7 million primordial follicles in her ovaries which are induced to 1 million at birth.

Most of these primordial follicles will never develop and mature.

During each menstrual cycle from 3 to 30 follicles are recruited and begin to grow and mature, but only one, the dominant follicle , reaches complete maturation, the others that had begun to mature with it, degenerate. Once the dominant follicle has reached complete maturation, it bursts, releasing the egg cell ready to be fertilized at the moment of ovulation.

What is the length of the menstrual cycle?

A menstrual cycle lasts an average of 28 days (25 to 36 days). There are women whose menstrual cycle length is very regular (the flow reappears after exactly the same number of days) and others who have it very variable.

The menstrual cycle is due to cyclical hormonal changes that occur in the woman’s body and which we now see. In particular, these changes are aimed at arriving at the time of ovulation and therefore at almost fertile days in which she can become pregnant.

Phases of the menstrual cycle

There are 4 main phases of the menstrual cycle and the most important from a reproductive point of view is the ovulation phase . With a little attention to the signals that the body sends it is possible to be able to identify it , even if it is not as clear and obvious as that of the flow.

Menstrual phase

The menstrual phase or menstruation or even basal stage, are “those days” in which we have blood flow. Menstrual losses last an average of 5 days, during which the  endometrium flakes off .

The flaking of the endometrium is determined by the decrease in the ovarian production of two hormones: estrogen and progesterone. This hormonal decline is due to the cessation of hormone production by the corpus luteum when pregnancy does not occur.

Menstrual blood losses consist on average of 30ml (the normal range is between 13 and 80ml) and are usually more abundant on the second day.

The first day of the menstrual cycle coincides with the first day of flow (= day 1).

Follicular phase

The follicular phase is also called the  proliferative phase , and can vary greatly in length.

In the first part of this phase, follicles are recruited and begin to grow under the influence of the hormone FSH. In the second part of this phase, a follicle is selected, called the dominant follicle, which, again under hormonal influence, matures and grows, reaching 18-20 mm before ovulation.


Ovulation is the moment when the  mature follicle bursts under the influence of the hormone LH, to release the egg cell.

The mature egg is released into the fallopian tubes (also called oviducts) and is ready to be fertilized.

Ovulation occurs approximately 38 hours after the start of LH surge (and approximately 17 hours after the LH surge). This peak is detected by ovulation tests.

Luteal phase

The luteal phase (or luteal phase) is the post-ovulatory phase which, unlike the follicular phase, has a predetermined length. Therefore when a menstrual cycle is defined as irregular in length, it is because the follicular phase has a variable length.

Once the dominant follicle has burst and released the egg cell, it transforms into the corpus luteum which produces progesterone .

This hormone has various functions including that of making the endometrium more favorable to the implantation of any conceived embryo.

The luteal phase is normally between 11 and 16 days long, but each woman’s specific length in this range varies by up to a couple of days.

The \ produces progesterone in increasing quantities reaching the peak at about 6-8 days after ovulation (about 25 mg/day).

Progesterone has a thermogenic activity, i.e. it increases body temperature. For this reason, the basal body temperature increases by about 0.5°C after ovulation.

This increase in temperature is used by the basal body temperature method to help understand when ovulation has occurred.

If fertilization has not occurred, the copro luteum begins to degenerate about 10 days after ovulation. As a result, the levels of progesterone and estrogen in the blood gradually drop, leading to the flaking of the endometrium and a new menstrual flow which marks the beginning of a new cycle.

In the last days of the cycle, before the next flow, one can suffer from premenstrual syndrome .

In this phase of the cycle , spotting and small bleeding may occur  .

The endometrium

We talked about the thickening of the endometrium. The endometrium is a mucous membrane that lines the uterus cavity , and is the “soil” in which the fertilized egg nestles to develop.

This mucosa is formed by a layer of glandular epithelium and by mucosa directly adhering to the muscular tissue. The more superficial endometrium undergoes the modifications that are typical of the menstrual cycle.

Special cases

Menstruation always follows ovulation by 11-16 days. If your period does not occur or you are pregnant or ovulation has not occurred (these are cycles without ovulation or anovulatory .).

If ovulation does not occur, there may still be blood loss which is not real menstruation but losses due to hormonal changes.

The 28 day menstrual cycle explained day by day

What happens from the first day of menstruation to the day before the next ones?

Let’s consider a cycle with ovulation around day 16 (counting from the first day of menstruation)

Compare these phases with an Assisted Reproduction Cycle

Day 1

Beginning of menstruation

Day 3-4

The endometrium sloughed off during menstruation.

Now about 20 immature eggs for each ovary are ready to be stimulated.

Estrogen levels are very low in this period so there are no cervical secretions and there is a sensation of dryness in the vaginal area.

The hypothalamus begins to release gonadotropins (GnRH) which stimulate the hypothalamus which in turn releases follicle stimulating hormone (FSH)

FSH simply does what its name implies and that is, it stimulates the immature follicles of the ovaries.

From day 5

Growing follicles produce ever greater levels of estrogen. This can be observed given that the vaginal secretions increase correspondingly and become more stringy and elastic ( cervical mucus ). At first they are sticky white, then wet, transparent and elastic.

In the meantime, the dominant follicle is also selected which will proceed to maturation while all the others degenerate.

Day 13-14

When the amount of estrogen produced reaches a critical threshold, the hypothalamus responds by releasing a surge of LH that fully matures the follicle to burst and thus release the egg cell.

Day 15

Approximately 36-38 hours pass from the time LH is released into the circulation to ovulation.

The emptied follicle collapses into the corpus luteum . Thus begins the production of progesterone necessary to support the implantation of the possibly conceived embryo.

The cervix of the woman has the role of filter: in fact, it selects the strongest and fastest sperm that will be the ones that will win the great sperm race.

Day 18-21

If conception has occurred, the fertilized egg will implant itself in the uterus within a few days

Around day 28

A pregnancy test should come back positive if conception has occurred, approximately 14 days after ovulation.

Kathryn Barlow is an OB/GYN doctor, which is the medical specialty that deals with the care of women's reproductive health, including pregnancy and childbirth.

Obstetricians provide care to women during pregnancy, labor, and delivery, while gynecologists focus on the health of the female reproductive system, including the ovaries, uterus, vagina, and breasts. OB/GYN doctors are trained to provide medical and surgical care for a wide range of conditions related to women's reproductive health.

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