How Does the Menstrual Cycle Work?

last updated on: 03.06.2025 | Medical review by:

Mayara Belli

Nutritionist

The menstrual cycle is where the entire cycle of life begins. When a girl gets her period (menarche), it means that a life is now able to generate another life. Pregnancy depends on ovulation to happen, and ovulation in turn depends on sexual intercourse for pregnancy to occur, right?

But how does the menstrual cycle work, and how can you use the cycle dates to get pregnant or avoid pregnancy? We can start to analyze how the menstrual cycle works through the length of a woman’s cycle, if there are any issues that keep it from occurring normally, and other factors. However, most of the time, the menstrual cycle has 3 phases:

Phases of the Menstrual Cycle

Follicular
Ovulatory
Luteal Phase

All phases of the menstrual cycle are stimulated by hormones, and they make all the “magic” happen. All these hormones are triggered by various parts of the reproductive system, but are effectively released into the body by the pituitary gland (a gland in the brain). The pituitary gland is responsible for almost all hormones in the body, in both men and women¹.

But How Does the Menstrual Cycle Actually Work?

The menstrual cycle starts with the release of a hormone called GnRH, which is secreted by the pituitary gland. This hormone then stimulates the production of another hormone called FSH (follicle stimulating hormone).

This FSH hormone makes the follicles start to grow and then gives way for another hormone to continue the ovulatory process. Besides this hormone, another one is necessary for the uterus to be ready for pregnancy. That’s where estrogen enters the scene, which is produced by the ovaries and plays a key role. Estrogen² helps the follicle mature for ovulation, and also thickens the endometrium enough so that pregnancy is viable. Without a proper endometrium, pregnancy will not happen, even if the egg is fertilized.

Mucus Characteristics

Estrogen is also responsible for the elastic mucus that appears during the ovulatory period. This mucus looks different because it is more elastic and slippery, similar to raw egg whites, or it may just be slippery and clear. That’s why knowing how the menstrual cycle works is so important; identifying the fertile mucus during this period is essential for those who want to get pregnant.

Ovulatory Phase

In the ovulatory phase, FSH gives way to estrogen, which then lets LH levels rise with the dominant follicles already mature. A drop in estrogen and a rise in LH—the luteinizing hormone—signals that mature eggs (one, two or three if they are mature) will be released, which is when ovulation occurs.

After the ovulatory phase, progesterone enters the scene. It is responsible for maintaining the endometrium, and if fertilization has occurred, this thicker endometrium can allow a pregnancy. High progesterone is responsible for pregnancy symptoms, and as its levels rise after ovulation, it can cause symptoms even with or without pregnancy. Progesterone produced in high amounts by the corpus luteum is also responsible for white, cream-like or milky-consistency mucus.

How to Calculate the Menstrual Cycle and Identify Ovulation?

Knowing how to calculate the menstrual cycle and identify ovulation is not always an easy task. Some women have periods that always come on the same date, while others have irregular cycles that change each month. One may be 30 days, the next 21 or even 40 days, so you never really know when your period will appear. However, you just need to learn to know your own body to identify ovulation to get pregnant or avoid being caught off guard by your period. Note down how many days between each period to figure out your cycle average.

Usually, ovulation happens 11 to 14 days before the next period. So, if a woman’s cycle is regular, this is quite easy. If the menstrual cycle is 28 days, the most fertile day will be the 14th day of the cycle. If it’s a 30-day cycle, the most fertile day is the 16th; in a 21-day cycle, day 8 is the most fertile. Remember, the fertile period lasts a week, so you should have sex 3 days before and 3 days after the most fertile day. But how do you calculate an irregular menstrual cycle?

Calculating an Irregular Menstrual Cycle

There are two ways to calculate an irregular menstrual cycle in women. The first is the Billings method. This method involves observing the appearance of cervical mucus during the cycle. The other way is by tracking basal body temperature. Both methods are very effective and can help women detect their ovulatory period more easily and increase the chances of pregnancy.

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