The menstrual cycle - more than just "the period”

Many of our biological functions are characterized by a recurring cycle. For example, the circadian rhythm (our internal clock) ensures that our digestion, the daily interplay of hormones and the sleep-wake rhythm are precisely coordinated within 24 hours. Women also have another cycle that is unique in nature - the menstrual cycle. In the course of puberty, a hormonal axis from the brain to the ovaries is formed specifically for this purpose. This very sophisticated control system is called the "hypothalamic-pituitary-ovarian axis" and will play a central role for about 40 years of your life. Every day, many different hormones communicate in precise coordination to regulate your cycle. This miracle of nature repeats itself in a recurring rhythm of about 29 days on average1 . Menstruation is the result and the most striking feature of this cycle.

It was not until 1923 that the Japanese gynecologist Dr. Kyūsaku Ogino made the observation that ovulation occurs at a relatively constant interval of about 12-16 days before bleeding, no matter how long the entire menstrual cycle lasts. Although the interaction of hormones was not yet known, the Dutch gynecologist van de Velden observed a little earlier - already in 1905 - a bi-phasic course of body temperature depending on the menstrual cycle. Further analyses have confirmed that the observed temperature rise is directly related to ovulation and the recurrent menstrual cycle.

Why do women have periods at all?

There are a variety of hypotheses as to why women menstruate. Insummary, science is certain that over the course of evolution, human fertility has continued to improve. It is a very successful concept of nature that the preparation for a possible pregnancy takes place at intervals of about a month. If fertilization does not take place, everything is started all over again. Menstruation is certainly anything but a defect of nature or a weakness, rather it is a sign of female power and the basis for the success of mankind, of which every woman (including you) should be proud.

The female cycle in detail

The menstrual cycle, also called the female cycle, is a complex process controlled by hormones from the brain, ovaries and uterus. It consists of two main phases divided into four sections, which are described in detail below:

  1. Menstruation: This stage is part of the follicular phase, usually lasts 3 to 7 days and begins with the first day of menstrual bleeding1 . During this period, the upper layer of the endometrium is shed and expelled from the body. Bleeding is triggered by a drop in hormone levels of estrogen and progesterone, which results in the inability to maintain the endometrium.

  2. Follicular phase: The follicular phase begins on the first day of menstruation and lasts on average about 16 days1 ,but can be longer or shorter. During this phase, several follicles in the ovaries are activated and begin to grow. Under the influence of the hormone FSH (follicle stimulating hormone) from the pituitary gland, the follicles produce estrogen, which causes the uterine lining to grow and thicken.

  3. Ovulation: Ovulation ends the follicular phase and usually occurs 12-15 days before the next menstruation2 . Under the influence of LH (luteinizing hormone) produced by the pituitary gland, a follicle containing a mature egg is selected. The follicle ruptures and releases the egg, which travels to the fallopian tube. This is the time when fertilization by a sperm cell can occur in the next 12 to a maximum of 18 hours. If fertilization takes place, the egg subsequently travels to the uterus, where it arrives after about 3 - 4 days. If fertilization does not take place, the egg cell dissolves.

  4. Luteal phase: After ovulation, the empty follicle is transformed into a gland called the corpus luteum, which produces progesterone. Progesterone prepares the uterine lining for a possible pregnancy and at the same time prevents follicles from maturing again. Progesterone is also responsible for raising basal body temperature by a few tenths of a degree during the second half of the menstrual cycle. This temperature difference is the most prominent sign of the luteal phase and serves Daysy as one of the bases for calculating non-fertile days during this period. The luteal phase usually lasts about 14 days. If fertilization has not occurred, progesterone levels drop at the end of the luteal phase and the menstrual phase begins again.


It is important to note that the menstrual cycle is different for every woman and can also be influenced by various factors such as stress, diet and physical activity. There may also be variations in the length of the cycle or in the duration of the individual phases. However, an individually regular menstrual cycle is an important indicator of women's health and can help identify problems related to fertility and reproductive health.

Let Daysy calculate your menstrual cycle for you!

Daysy is a state-of-the-art Fertility Tracker with a unique algorithm. Every morning Daysy needs your basal body temperature to calculate your fertility quickly and reliably. You take the measurement comfortably under your tongue before getting up. When the measurement is successfully completed, Daysy will beep twice after about 60 seconds. The result will be displayed immediately afterwards. On some days, Daysy will ask you if you are having your period. If not, you do not need to do anything. Daysy will automatically save all results for you.

Daysy's indicator is clear and simple: a green light means you are not currently fertile. A red light indicates that you are currently fertile/possibly fertile and could become pregnant through unprotected intercourse. A flashing red light indicates the predicted day of ovulation.On some days, especially during the first month of measurement, Daysy will show you a yellow light. This means that Daysy needs to get to know your cycle better or that you are having a slightly irregular cycle. You can treat "yellow days" as "red days" and further increase your chances of getting pregnant quickly.

If you want to view your data in addition to your current fertility status, you can install the free app "DaysyDay" on your smartphone and synchronize it with Daysy. This way you always have all your data with you and can preview when you are likely to have your next ovulation as well as your next menstruation.

Your menstrual cycle: as individual as you are!

If you use Daysy for a while, you will see that your menstrual cycle is very individual. Even though the order of the phases is the same for every woman, there are individual differences. For example, you may have a rather long menstrual period or a short cycle overall. The timing of ovulation can also vary from woman to woman. If you have a lot of stress at times, your cycle may reflect this.. Variations in your cycle are an excellent indicator of how your body is handling your current situation and can be indicative that it is time to reduce your stress level.

Daysy is an intelligent fertility tracker that lets you get to know your very own menstrual cycle.


  1. Niels van de Roemer, Liya Haile & Martin C. Koch (2021) The performance of a fertility tracking device, The European Journal of Contraception & Reproductive Health Care, 26:2, 111-118.

  2. Lenton EA, LANDGREN B, Sexton L. Normal variation in the length of the luteal phase of the menstrual cycle: identification of the short luteal phase. BJOG: An International Journal of Obstetrics & Gynaecology. 1984 Jul 1;91(7):685-9.

Daysy is an intelligent fertility tracker that lets you get to know your very own menstrual cycle.

Further articles on the topic of Menstrual Cycle in our adviser:

Menstrual cycle calculator

Daysy is an intelligent fertility tracker that lets you get to know your very own menstrual cycle.