The fertility window, that is, the period during which it is possible to conceive, lasts only a few short days during the cycle. When you want to have a child, it is important to identify these fertile days. The period calendar can be used as a first-line treatment, but in the end it does not prove to be an unreliable tool.

The best way to put your chances on conceiving a baby is to have sex during the fertile period of the cycle. However, this window of fertility lasts barely 3 days during the cycle. It begins about 2 days before ovulation (the average period during which sperm can survive in a woman’s body) and ends less than 1 day after ovulation (the egg has a lifespan of 10 at 15 p.m. after ovulation).

The period calendar can be a first step in trying to detect when you ovulate. It allows to calculate the date of ovulation according to the duration of the cycle, based on this physiological fact: on an ovarian cycle of 28 days (average duration of a cycle), ovulation takes place around the 14th day. We also know that it is on the first part that the variations at the origin of the different cycle times occur. The second part of the cycle (luteal phase), which begins just after ovulation, has a generally constant duration of 14 days.

The principle is simple: it is a question of noting on a calendar the date of the first day of the rules. It corresponds to the first day of the cycle. We then note the date of the first day of the following periods (ie the start of the next cycle). These two dates make it possible to know the duration of the cycle, in days.

To know the duration of the first phase of the cycle and the date of ovulation, it suffices to subtract 14 (average duration of the luteal phase) from the duration of the cycle. To calculate the date of ovulation on the next cycle, we will then postpone this calculation starting from the first day of the rules.

Some examples :

• period date: November 6 / next period date: December 4. Cycle length: 28 days. 28 – 14 = 14 (duration of the first phase of the cycle). Ovulation date on the following cycle: December 4 + 14 days = December 28.
• date of menstruation: October 1 / date of next period: November 1. Cycle length: 31 days. 31 – 14 = 17 (duration of the first phase of the cycle). Ovulation date on the following cycle: November 1 + 17 days = November 18.

The period calendar method of finding out when you ovulate has its limitations. It requires having regular cycles, but this is far from the case for all women. Moreover, even with regular cycles of 28 days, ovulation does not necessarily occur on the 14th day. Directly dependent on hormonal secretions governed by the hypothalamic-pituitary axis, many factors (stress, diet, emotional shock, intensive sport, etc.). can indeed disrupt ovarian cycles, and change the date of ovulation.

Other methods exist to define the date of ovulation:

• the temperature curve: This method is based on this physiological fact: ovulation causes a strong secretion of progesterone, certain derivatives of which have the effect of increasing body temperature. After ovulation, the temperature rises by several tenths of a degree (3 to 5), to generally exceed 37 ° C. The temperature curve therefore consists of taking its temperature every morning before getting up, and plotting the results on a curve in order to detect this thermal plateau. The presence of this plateau makes it possible on the one hand to confirm the existence of an ovulation, on the other hand to date this ovulation (ovulation taking place the day before the rise in temperature).
• observation of cervical mucus (or Billing method): This method is based on another physiological fact: the changes in cervical mucus in quantity and texture throughout the ovarian cycle. As ovulation approaches, the mucus increases in quantity and becomes more fluid and stringy to promote the passage of sperm. On the day of ovulation, its texture is comparable to that of an egg white: fluid, elastic, shiny and transparent. The idea is therefore to detect the period of ovulation by appreciating daily, using his fingers, the consistency of his cervical mucus.
• the sympto-thermal method: combines the temperature curve, the observation of the cervical mucus and the observation of the cervix which, as ovulation approaches, softens and opens in order to promote the passage of sperm.
• ovulation tests: These over-the-counter tests are based on the detection of luteinizing hormone (LH) in the urine, and more specifically the LH surge. This hormone is indeed produced in greater quantity 24 to 36 hours before ovulation, and it is precisely this LH surge that will trigger ovulation. There is no need to start these tests on the first day of the cycle. For a 28-day cycle, it is recommended to start the tests on the 10th day (28-18), i.e. 10 days after the start of menstruation. If no ovulation has been detected after 10 days of testing, testing will be restarted in the next cycle starting two days earlier. This method is, by far, the most reliable and the simplest to detect the date of ovulation.

Implantation refers to this crucial stage in the beginning of pregnancy during which the egg fertilized by the sperm gradually implants itself in the uterine lining. In theory, implantation takes place around 7 to 8 days after fertilization (1), while the egg migrates from the tube to the uterus.

Regarding the date of fertilization, if sexual intercourse took place before or on the day of ovulation (or at least the supposed day of ovulation), in theory fertilization took place on the day of ovulation. We will therefore add 7 days to the date of ovulation in order to know the date of implantation.

However, many variables go into the calculation of the implantation date: the ovulation date, the fertilization date, the length of the period between fertilization and implantation. The period calendar appears ultimately unreliable to know this date which is, moreover, not really important to know for the good progress of the pregnancy.