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Ovulation and Fertile Days Calculator

Reviewed by MD Katiuska Ríos Calderón

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When did your last period start?

Average length of your menstrual cycles:


The period begins
Ovulation day (very fertile)
Most fertile days

Optimal fertility period:

  • From to

More details of interest:

  • Approximate ovulation date:
  • Approximate date of the next menstruation:
  • If you become pregnant during this period, the due date could be:

Related Resources

Table of Contents

How to Use the Fertile Days Calculator

If you've arrived here, it means you're eager to calculate your fertile days, and we're going to give you two options to resolve your doubts.

You can use our fertility calculator to perform this calculation automatically and completely free, or, on the other hand, you can learn to find your fertile days manually, taking into account some of the variables, such as the regularity or irregularity of your menstrual cycle.

If you prefer the second option, keep reading and discover everything you need to know to calculate your fertility.

Menstrual Cycle: What It Is and How Ovulation Works

The menstrual cycle is a natural and regular process that groups certain changes produced in the female reproductive system, ranging from the first day of menstrual bleeding, through the ovulatory period, preparing the body for pregnancy, and culminating again with the first day of the next cycle in case you have not become pregnant, hence it is a cycle.

As we mentioned, the first day is the one on which menstruation appears.

After about 14 days (assuming the approximation to 28-day menstrual cycles) ovulation takes place, that is, the release of the egg from the follicle and the beginning of its journey towards the uterus. This corresponds to the ovulatory cycle.

Immediately after ovulation, the body begins to prepare for the arrival of the new menstrual cycle, normally requiring around 2 weeks of organization before its new appearance.

Here the so-called premenstrual syndrome manifests, whose symptoms can vary greatly from woman to woman, ranging from a slight retention of fluids, premenstrual weight gain, and mood changes among others.

Once the cycle has ended, it starts again and repeats several times throughout your reproductive life until you reach menopause.

And why do I want to know this? Well, because our fertile days calculator will indicate several dates.

Firstly, the days when your next menstrual period should appear, and secondly, those that should be your most fertile days.

Knowing which are the most fertile days of the menstrual cycle allows you to get an idea of when it is best to have sexual intercourse if your goal is to become pregnant, as those fertile days your chances of pregnancy will increase.

The Preovulatory Phase

The preparations for the fertile phase begin with the preovulatory period.

During this phase, the pituitary gland, is activated to produce gonadotropins, substances that stimulate the development of the follicles, these are similar to small pouches that are in the ovaries and that wrap or contain the eggs.

At the same time and as a result of the production of gonadotropic hormones (FSH and LH), the production of estrogens begins to increase, which are the hormones responsible for the growth of the uterine mucosa whose production will only decrease at the time of ovulation.

About halfway through the cycle, the pituitary gland begins to produce a particular type of gonadotropin, called Luteinizing Hormone or LH, which causes the follicle to break to release the egg intended to be fertilized and subsequently nest in the endometrium

The 6 Fertile Days

After the egg is released, from the remains of the opened follicle the so-called corpus luteum is formed, which produces progesterone.

This begins your ovulation phase, which lasts about 24 hours.

However, the period in which it is possible to become pregnant is wider because sperm live around 72 hours inside the female genital tracts, thanks to this it is possible to become pregnant during a period of approximately 6 days a month.

In this case, constantly calculate your fertile days to increase your chances of pregnancy.

On the other hand, the probabilities increase if you make love in the preovulatory phase, as long as the male sperm are sufficiently vital to remain active upon contact with the egg.

The most fertile days are undoubtedly those immediately before ovulation and the same day the egg enters the fallopian tube.

How to Calculate Fertile Days

The fertile period is part of the ovulation process, which normally occurs 14 days before menstruation, assuming 28-day menstrual cycles. However, it is not so exact, as on many occasions, stress or even a cold, can change that day.

The cycles are not always 28 days, sometimes a woman can have long cycles, lasting 32-35 days, or shorter cycles, lasting only 21 or 25 days, for example.

In a 24-day cycle, ovulation will be more or less 9 to 11 days after the first day of menstruation and your fertile days will be that same day and the 3 previous and following days.

If, on the other hand, you have a cycle of, for example, 32 to 35 days, your ovulation would occur 16 to 18 days after the previous menstruation.

Ovulation Calculation with Cervical Mucus

If your cycles are not very regular, this method is a good option. The cervical mucus method or known as Billings method (MOB) was first described in Australia and was presented as a method for family planning.

It is a good option to consider as another possibility to discover your fertile days, undoubtedly the best of it is based on one of the most evident anatomical and physiological manifestations such as the modification of cervical mucus.

This method is based on identifying the patterns of potential fertility and obvious infertility within the cycle, regardless of how long the cycle lasts

It is a substance produced daily by the cervix of the woman but that increases substantially when ovulation approaches.

In fact, its main function is to create a favorable environment for the transit of sperm, favoring conception.

The characteristics of cervical mucus will help you know when the time of your ovulation has arrived. One of the fundamental aspects to consider is its consistency, depending on this you will know if you are ovulating or not:

If it is excessively sticky it means that you are not ovulating yet. This happens just after your menstruation has passed.

If, on the other hand, it has a somewhat creamier and whitish touch, it means that your ovulation period is approaching, but it has not yet arrived.

And if finally the secretion of the mucus is fluid, similar to egg white and stretches between your fingers about 10 centimeters approximately without breaking, then almost certainly you are ovulating and therefore it can be a magnificent moment to try to get pregnant.

Calculation with Basal Temperature

Another widely used method, although perhaps more cumbersome, is the daily measurement of basal temperature.

To discover your basal temperature, you can do it either orally, rectally or vaginally and, when you see that the temperature rises compared to the previous days it means that the period or moment of ovulation is approaching.

The elevation of your body's basal temperature identifies the possibility of fertility. The luteinizing hormone, which stimulates ovulation, causes an increase in temperature, from 0.3 to 0.6 ° C (degrees Celsius) in your basal temperature.

You must do it always before getting up and at the same time and have slept at least 6 hours. It is estimated you have ovulated after observing 3 consecutive days of elevated temperature and when you notice that your temperature starts to rise, it is a moment to have intercourse.

For this method a little more imprecise, perhaps you need to keep a sort of diary where you note all these temperatures for a couple of months to get to guess without errors which will be the moment when your ovulation will start.

This is everything you need to know about how to calculate your fertile days to try to get pregnant, now it only remains to apply these tips and enjoy the experience.

Summary of What We've Learned

  • Firstly, how to use our ovulation and fertile days calculator to perform the process automatically.
  • Secondly, we have explained the functioning of the menstrual cycle and how to calculate it manually, whether you have a regular menstruation or an irregular one.
  • Lastly, we have also taught you two methods to calculate your fertile days if you are irregular: the control of cervical mucus and basal temperature.

If you liked both the article and the online fertility simulator, don't forget to share it with your contacts on your social networks.

Finally, if you find faults or technical deficiencies, want to make a proposal or simply want to encourage us, make use of the contact page.

Bibliographic Resources

  • Hambridge HL1, Mumford SL, Mattison DR, Ye A, Pollack AZ, Bloom MS, Mendola P et al. The influence of sporadic anovulation on hormone levels in ovulatory cycles. Hum Reprod. 2013 Jun;28(6):1687-94. Available at
  • Brzyski R, Knudtson J. Menstrual Cycle [Internet]. MSD Manual. Available at:
  • Pallone S, Bergus G. Fertility Awareness-Based Methods: Another Option for Family Planning. The Journal of the American Board of Family Medicine. 2009;22(2):147-157. Available at:
  • Butler Tobah Y. Signs of Ovulation: When is Conception Most Likely? [Internet]. Mayo Clinic. 2016. Available at:
MD Katiuska Ríos Calderón
Doctor of Medicine and Surgery from UCLA. Specialized in Gynecology and Reproduction. Subspecialized in Endocrine Gynecology and Climacteric.
Dr. Katiuska Ríos Calderón holds a degree in Medicine from the Universidad Centr... Read more »

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