How do ovulation calculators work out your most fertile period?

By (gynecologist), (embryologist), (embryologist) and (biochemist).
Last Update: 09/13/2022

When planning a pregnancy, knowledge of our own body and menstrual cycles is very useful to maximise the chances of achieving a pregnancy in a natural way.

Fertility calculators can be useful when you are begin trying to get pregnant. However, if you haven´t had any success after trying to conceive for over a year of unprotected sexual intercourse, or after 6 months if the woman is older than 35, it is a good idea to consult an infertility specialist.

Menstrual cycle and ovulation

A woman's menstrual cycle begins with the onset of menstruation, i.e. the first day of menstrual bleeding corresponds to day 1 of the cycle. Therefore, by counting the days from the start of one period until the next one, a woman will know the length of her menstrual cycle.

The average length of the menstrual cycle is 28 days, but there are many women who have a shorter or longer one. In general, ovulation (the release of the egg from the ovary) occurs 14 days before menstruation. This is why we often say that ovulation occurs on day 14 of the menstrual cycle, as this would be the case in a 28-day cycle.

Nevertheless, if a woman has a menstrual cycle shorter than 28 days, ovulation would occur before day 14. Likewise, if the menstrual cycle is longer, ovulation will occur later.

For example, in a short 26-day cycle, a woman might ovulate on day 12 (a simple calculation of 26-14). In contrast, with a 30-day cycle, a woman might ovulate on day 16.

Ovulation and fertile days

Ovulation is a key moment when trying to conceive. As mentioned above, with ovulation the egg is released from the ovary and collected by the Fallopian tubes.

In order for pregnancy to occur, the egg must meet the sperm in the Fallopian tubes. As such, the days close to ovulation (periovulatory period) are the days on which this encounter is most likely to occur if sexual intercourse takes place and, therefore, are known as fertile days.

However, these days are determined by the different survival times of the egg and sperm. While sperm can be viable for up to 5 days in the female reproductive tract, the egg can be fertilized only in the first 12-24 hours after ovulation. Therefore, the fertile period ranges from about five days before ovulation to one day after.

How fertility calculators work

A fertility calculator makes it easy to perform the calculations discussed above. Hence, a woman can easily identify her most fertile period.

To use a fertility calculator, you only need to enter the date on which the last menstrual period (LMP) began and the length of your menstrual cycles. With these data, the calculator estimates the day of ovulation and the most fertile days of the cycle: the dates on which there is a greater probability of achieving a pregnancy.

So, if you are looking for a pregnancy, you should have sexual intercourse on the indicated days (always bearing in mind that this is an estimate). Nevertheless, it is important not to make sex an obligation.

In fact, having intercourse too many times in a row could be counterproductive in the search for pregnancy, since the seminal quality would be reduced. Trying for a baby should not become something scheduled, but ideally, sexual intercourse should take place every two days or so during a woman's fertile days. However, it should be taken into account that a very long previous abstinence period would also affect seminal quality.

Effectiveness of fertility calculators

Ovulation and fertility calculators are very useful tools when actively trying to get pregnant. These tools allow women to better understand their menstrual cycle and make a personalized fertility calendar, which can increase their chances of pregnancy.

It is important to mentione that these calculators is more reliable when the woman has a regular menstrual cycle, with a stable duration between 21 and 35 days. However, there are several factors that can influence ovulation. This means the moment in which the egg is released may vary by a few days with respect to the result of the calculator, even if you have regular menstrual cycles.

Therefore, the woman should interpret the result as an estimate of the ovulatory period, which is more accurate if cycles are regular, but may not be exact.

For this reason, there are certain fertility signs that are useful to know. These can help a woman, along with the ovulation calendar, to more accureatly know her most fertile days. Among these signs are:

In addition, a woman may also notice other symptoms during her ovulatory period that help her identify when she is ovulating. These symptoms may include some abdominal pain and even an increase in libido.

Finally, ovulation tests detect the LH hormone surge about 36 hours before ovulation takes place. These tests, which are easily purchased in pharmacies, are also very useful in the search for pregnancy.

You can read more about these other methods for predicting ovulation in the following article: What are the signs of fertility in women?

Fertility calculators with irregular cycles

If you have an irregular menstrual cycle it is more complicated to work out the most fertile days. In these cases the ovulation calculator will not be accurate, so it may be better to pay attention to the fertility signs mentioned above.

Notwithstanding, if a woman has irregular menstrual cycles, it is advisable to see a gynecologist so that the causes can be evaluated. Thus, depending on the diagnosis, the specialist will be able to provide the appropriate treatment and/or give the appropriate advice to the woman.

FAQs from users

Can ovulation timing vary with age?

By Guillermo Quea Campos M.D. (gynecologist).

Yes, the timing of ovulation changes with age. During reproductive life, women usually have regular menstrual cycles and ovulate each month. These eggs mature during the menstrual cycle until ovulation occurs. This process, although very simple to explain, is quite complex because many substances are involved that will favor recruitment and dominance selection, causing the ovarian reserve to decrease throughout the reproductive life.

We know that the ovarian reserve begins to decrease from the age of 35 years. From 38-39 years of age, in addition to the decrease in ovarian reserve, the quality of the eggs also decreases. Therefore, the chances of pregnancy are compromised.

Because of this, the time of ovulation will change with age according to the ovarian reserve, in some cases becoming anovulatory cycles and finally disappearing menstruation and entering the menopausal/climacteric period.

Do ovulation calculators serve as a method of contraception?

By Silvia Azaña Gutiérrez B.Sc., M.Sc. (embryologist).

No, ovulation and fertility calculators should not be used as a method of contraception.

These tools are useful in trying to conceive, as they provide an estimate of the period in which pregnancy is most likely to occur. However, the woman's fertile days may vary from the result estimated by the calculator (even more so if the woman has irregular cycles), so the woman could become pregnant outside the days that the calculator has indicated as "fertile".

Therefore, to avoid pregnancy, a contraceptive method should always be used. In addition, if it is a barrier method of contraception such as a condom, it will also prevent the spread of sexually transmitted diseases (STDs).

What signs of fertility are there in men?

By Silvia Azaña Gutiérrez B.Sc., M.Sc. (embryologist).

Men, lacking a menstrual cycle, do not have more or less fertile days.

However, when trying to get pregnant, it is recommended to space sexual intercourse about 2 days apart. More or less abstinence could affect seminal quality.

Is it possible to use the fertile days calculator to have a baby girl?

By Silvia Azaña Gutiérrez B.Sc., M.Sc. (embryologist).

There is a method, called the Shettles method, according to which it is possible to become pregnant with a boy or a girl depending on the time at which conception takes place.

According to this method, X sperm (which would give rise to a girl) are slower, but more resistant to vaginal acidity than Y sperm (which would give rise to a boy). Thus, the couple should schedule sexual intercourse up to 3 days before ovulation to favor having a girl, or on the day of ovulation to have a boy.

However, it is important to mention that the effectiveness of this method is quite questioned.

As we have mentioned, having a good knowledge of the menstrual cycle and its phases can help us increase the chances of pregnancy. Therefore, if you want to read more information about the menstrual cycle, we recommend visiting the following link: The different phases of the menstrual cycle.

Alternatively, if you want to know more about ovulation tests, you can read up with the following article: Ovulation tests: how do they work and what is their purpose??

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FAQs from users: 'Can ovulation timing vary with age?', 'Do ovulation calculators serve as a method of contraception?', 'What signs of fertility are there in men?' and 'Is it possible to use the fertile days calculator to have a baby girl?'.

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Authors and contributors

 Guillermo Quea Campos
Guillermo Quea Campos
Guillermo Quea, MD has a degree in Medicine and Surgery from the University of San Martin de Porres. He also has a Master's Degree in Human Reproduction from the Universidad Rey Juan Carlos and another in Public Health and Preventive Medicine from the Universidad del País Vasco. More information about Guillermo Quea Campos
Member number: 282860962
 Laura Garrido
Laura Garrido
B.Sc., M.Sc.
Bachelor's Degree in Biotechnology from the Pablo de Olavide University (UPO) of Seville, Spain. Master's Degree in Biotechnology of Human Assisted Reproduction from the University of Valencia (UV) and the Valencian Infertility Institute (IVI). Experience at IVF, andrology, and general analysis laboratories. Embryologist specialized in Assisted Reproduction. More information about Laura Garrido
 Silvia Azaña Gutiérrez
Silvia Azaña Gutiérrez
B.Sc., M.Sc.
Graduate in Health Biology from the University of Alcalá and specialized in Clinical Genetics from the same university. Master in Assisted Reproduction by the University of Valencia in collaboration with IVI clinics. More information about Silvia Azaña Gutiérrez
License: 3435-CV
Adapted into english by:
 Michelle Lorraine Embleton
Michelle Lorraine Embleton
B.Sc. Ph.D.
PhD in Biochemistry, University of Bristol, UK, specialising in DNA : protein intereactions. BSc honours degree in Molecular Biology, Univerisity of Bristol. Translation and editing of scientific and medical literature.
More information about Michelle Lorraine Embleton

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