Menstrual cycle (ovulation, irregular): learn how to calculate

All women of childbearing potential have to deal with menstruation once a month. This time interval between one menstruation and another can be called the Menstrual Cycle, which is a long process involving several hormones and natural changes in the body.

Understand more in the text below!

Index – in this article you will find the following information:

  1. What is the menstrual cycle?
  2. Stages of the menstrual cycle
  3. What is an irregular menstrual cycle?
  4. Why is menstruation irregular?
  5. How to calculate the menstrual cycle?
  6. Menstrual cycle table
  7. Signs that indicate the fertile period
  8. Common questions

What is the menstrual cycle?

Menarche, which is the woman’s first menstruation and occurs between 10 and 15 years of age, marks the beginning of reproductive life and the 1st menstrual cycle.

This first menstruation only happens when all parts of the reproductive system are properly matured and functioning together.

Thereafter, women experience bleeding called menstruation, which usually occurs on a monthly basis. We could then say that the menstrual cycle is the time between bleeding and bleeding.

Technically, menstruation can be categorized as loosening of the endometrium, a tissue of the uterus irrigated by blood vessels and specialized glands that has the function of welcoming and nourishing the embryo in the first moments of pregnancy.

The endometrium is like the “bed” where the embryo, after fertilization, will lodge and receive nutrients. During the menstrual cycle, the endometrium thickens in case the woman becomes pregnant. If fertilization does not happen, the endometrium is eliminated in the form of menstruation.

This physiological process is controlled by hormones that cause changes within the womb of women, more specifically on the endometrium, and lasts between 21 and 35 days, with an average of 28 days. Menstrual flow lasts, on average, from 2 to 6 days and represents a total blood loss of 20mL to 60mL.

Unlike men, who produce new sperm at each intercourse, women are born with a fixed number of eggs for the rest of their lives. At the time of birth, there are about 1 to 2 million, but that number is decreasing and, by the age of 12 or 13, only around 400,000 to 500,000 remain.

As time goes on, they continue to leave in large numbers. With each menstrual cycle, the female body makes available, on average, a thousand potential eggs, but only one actually develops.

Even taking the birth control pill, this monthly decrease in the number of eggs continues to occur and nothing can be done to stop the process. Over time, the ovary empties until, in the late 30s and early 40s, the stock is practically zeroed.

But defining the menstrual cycle like this is to simplify the process. It is actually a direct relationship between the brain and the female reproductive system in an intricate hormonal chain.

Stages of the menstrual cycle

The menstrual cycle can be divided into 3 phases:

Follicular phase

The follicular phase represents the beginning of the cycle, which is the first day of menstruation, and lasts between 9 and 23 days, with an average of 15 days. The brain produces the Follicle Stimulating Hormone, FSH, which stimulates the follicles in the ovaries to turn into eggs.

It works like this: the action of FSH stimulates several follicles (on average, between 3 and 30), however, only one of them, called the dominant follicle, develops and becomes a mature egg.

In addition, FSH promotes the release of greater amounts of estrogen from the ovary, another hormone responsible for making the lining of the uterus ready for a pregnancy, that is, estrogen causes the endometrium to thicken.

Ovulatory phase

During the ovulatory phase, estrogen levels continue to rise, causing the woman’s body to produce the luteinizing hormone (LH), which is responsible for selecting the dominant follicle, the one that will become a mature egg.

Around the 14th day of the cycle (counted from the first day of menstruation), through the action of LH in the membrane of the follicle, there is the rupture and release of the ovary into the tubes, which characterizes the period of ovulation.

It is at this point that the clearest signs of ovulation happen. There is a vaginal mucus of a color and consistency similar to that of egg white, in addition to the famous stitches on one or both sides of the lower abdomen.

In general, the eggs survive 24 hours outside the ovaries until they are fertilized by a sperm or not. A sperm can survive for up to 5 days in the woman’s body, so it is possible to become pregnant even when sexual intercourse occurred up to 5 days before ovulation.

Luteal phase

The luteal phase usually happens in the last 12 days of the menstrual cycle. During this time, the follicles that remain in the ovaries undergo changes and turn yellow, receiving the name of corpus luteum .

They have the function of starting to produce progesterone and estrogen in large quantities. This makes the endometrium suitable for a possible pregnancy and helps stop LH and FSH production.

At this stage there is also a greater production of estrogen, which causes some women to experience increased breast sensitivity, mood swings and swelling.

When fertilization does not occur, LH levels decrease, causing the corpus luteum to shrink and estrogen and progesterone levels decrease until the endometrium is eliminated (menstrual bleeding), beginning the next cycle.

On the other hand, when fertilization takes place, the egg is stuck to the endometrium and the woman’s body begins to produce hCG, a hormone that keeps the follicle producing high levels of estrogen and progesterone to maintain the lining of the uterus until the formation of the placenta occurs. .

Read more: How do I know if I am pregnant? Pregnancy symptoms and diagnosis

What is an irregular menstrual cycle?

As we have seen, the normal cycle lasts between 21 and 35 days. The irregular cycle, on the other hand, is one that presents significant changes every month or in most of them.

Technically, this means a cycle with menstruation lasting more than 8 days in a row, with intervals greater than 35 or less than 21 days (depending on your regular cycle) and with great blood loss (when menstruation is able to soak more than 3 days) absorbents per day).

It is also possible to characterize an irregular cycle when there are variations in normal duration, that is, when bleeding lasts 2 days in one month, in the other it lasts 6 and in the other 5, for example.

It is worth remembering, however, that some changes in the cycle are normal and are not always signs of a problem or sign of pregnancy. In any case, it is always good to consult the gynecologist and clarify doubts.

Read more: Pregnancy Test (blood, pharmacy): how to do it, price, results

Why is menstruation irregular?

Irregular menstruation can happen for a number of reasons, but it is more common in women between 10 and 20 years old, because in this age group the hypothalamus-pituitary-ovary axis is still immature, resulting in menstrual cycles without the production of eggs.

This occurs in approximately 50% of adolescents during the first years in a row after the first menstruation, but tends to stabilize naturally.

Other than that, the main causes for an irregular menstrual cycle are:

  • Post-pregnancy period;
  • Early menopause;
  • Polycystic ovary syndrome;
  • Thyroid disorders, such as hyperthyroidism;
  • Endometrial polyp;
  • Obesity;
  • Eating disorders that cause excess weight loss, such as anorexia nervosa;
  • Excessive physical activity, especially in female athletes;
  • Change of contraceptive;
  • Stress and emotional disorders.

How to calculate the menstrual cycle?

To calculate the menstrual cycle, note the 1st day of menstruation, which corresponds to the 1st day of the menstrual cycle, and from there, the woman must count the following days until the 1st of the next menstruation. Typically, it varies between 21 and 35 days.

When the cycle time is less than 21 days or greater than 35 days, you may have an irregular menstrual cycle, which can be a sign of problems. In such cases, it is always a good option to talk to the gynecologist.

Menstrual cycle table

The menstrual cycle table is a method widely used by women seeking to conceive, as it helps to find out more easily when the fertile period will occur . Despite this, it is worth remembering that it should not be used to prevent pregnancy .

To make your own table, you must know your menstrual cycle. In general, ovulation occurs on the 14th day after menstruation, and this is considered the most fertile day for women.

However, as the exact day of ovulation can still vary due to several factors, such as stress and hormonal changes, one must consider 2 days before and two days after that 14th day, totaling 5 days – also called the fertile period .

To make the table more efficient, it is recommended that the woman write down in a calendar every day that she is menstruating for at least 1 year and thus be able to observe regularities.

Signs that indicate the fertile period

It is possible to know the fertile period by monitoring the menstrual cycle. With it, you can have an estimate of the day when ovulation occurs and know if you are in the fertile period or not.

Ovulation occurs 14 days before menstruation and is the time when a woman is most fertile. The fertile period is around 2 days before and 2 days after the day of ovulation, which represents approximately 5 fertile days.

But there are also other ways to know if you are in the fertile period or not, as there are some very characteristic symptoms. Among them are:

  • Vaginal secretion (filance): during the period of ovulation, there is the release of a hormone called estradiol, which causes the vagina to produce more fluid than usual, as if it were a lubrication mechanism that helps sperm to enter. The fluid is transparent and elastic, similar to egg white.
  • Emergence of acne (pimples): one of the signs that one is in the fertile period is an increase in the oiliness of the skin and the appearance of small pimples or blackheads;
  • Slight increase in temperature: the increase in progesterone can cause an increase in body temperature between 0.3 ºC and 0.8ºC. You can take your temperature right when you wake up.
  • Increased libido and appetite: during the fertile period, hormones are on the skin, causing the appetite (gastronomic and sexual) to be elevated.

Read more: Gestational Calculator (weeks, months): how to calculate, ultrasound

Common questions

When does the menstrual cycle begin?

The menstrual cycle starts from the day of the first menstruation and lasts between 21 to 35 days.

Does contraceptive change menstrual flow?

The woman’s menstrual cycle may change due to hormonal contraceptive treatments. The use of the pill, vaginal ring and implants makes the body believe that it no longer needs to produce progesterone, which makes menstrual flow lighter.

In addition, other factors may be involved in changing the cycle, such as disease. Cancer , heart disorders and thyroid problems, for example, can cause changes in the regularity of bleeding.

However, it is worth mentioning that it is not because your menstrual period is unregulated or abnormal that you have signs of a disease or dysfunction. Before assuming anything, talk to your gynecologist.

Are menstrual cycles always the same?

It depends . In the first 2 or 3 years after the first menstruation, it is normal for the cycles to be irregular. From that time on, menstruation begins to stabilize. In the case of women who already had an irregular cycle and had a child, after the first delivery, they tend to stabilize.

Bearing in mind that around the age of 45, menstrual regularity may begin to suffer variations, as women approach menopause .

Does a woman not get pregnant when she is menstruating?

False! Although the chance of pregnancy during menstruation is quite small, it is still possible for the egg to be fertilized.

For a pregnancy to happen during menstruation, it is necessary that the menstrual cycle is very short, so that the date of ovulation occurs close to that of the initial bleeding. Another possibility is that bleeding lasts a long time, causing ovulation to occur at the same time.

That is, although rare, it can happen. For this reason, condom use should always be prioritized .

Does continuous contraception without pause cause infertility?

No! Contraceptive use is unlikely to affect women’s fertility. What can happen is that the woman’s body is used to the medication, making it necessary (sometimes long) for the cycle to normalize after stopping use.

Is sex during menstruation more pleasurable?

It depends . There is an idea that women feel more pleasure when they are menstruating, but this is not always true. During menstruation, testosterone levels are higher, which can cause sexual desire to be increased, but by itself it does not make sex more or less pleasurable.

Is exercising during your menstrual period bad for you?

No! Quite the opposite! In fact, there is a benefit of physical activity in releasing hormones. There are no contraindications for physical exercise during the menstrual period of a healthy woman.

Exercises can even help relieve colic thanks to the endorphin (well-being hormone) produced after physical activity.

Does menstruation stop when you are in the water?

Myth! In general, menstrual bleeding does not have a large volume, being a continuous and small flow. Therefore, when the woman enters the water, the trickle of blood that flows tends to be diluted in the water and not be noticed. But he is not interrupted.

However, if the water is cold, there may be an effect on the blood vessels, causing them to constrict, making it harder and slowing the blood out.


Calculating the menstrual cycle may seem like a difficult task, but it is actually simpler than many realize.

Find out more information about women’s health in the Healthy Minute!

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