How to Calculate Your Fertile Days

Whether you want to get pregnant or you want to avoid it, it's always helpful knowing how to calculate your fertile days. However, it's not always easy and it's not exact. 

Reviewed and approved by the doctor Maricela Jiménez López.

Written by Thady Carabaño

Last update: 15 December, 2022

In order to get pregnant, you need to know which days you’re fertile. The same goes for avoiding pregnancy. However, determining fertile days with 100% accuracy is impossible. Remember, calculating fertile days is always an approximation. 

How to Calculate Your Fertile Days

The menstrual cycle is the interval of time that passes between two menstrual periods. The first day of menstruation is the beginning the a new cycle. Each woman has an individual cycle, which can vary from month to month.

In addition, the duration of a woman’s cycle usually changes throughout her life. Your cycle may have lasted 26 days when you were 18, and ten years later it could change and last longer, or less time. This will also change after pregnancies.

Having irregular menstrual cycles isn’t a cause for concern. It’s normal. Just know that you’ll need to be more attentive to your cycle if you want to get pregnant, or if you think you may have amenorrhea (absence of menstruation) if you aren’t pregnant.

When Does Ovulation Occur?

In a cycle of 28 days, which is the normal length of the menstrual cycle, ovulation will take place on day 14 of the cycle, approximately. This means that the fertile days in a regular menstrual cycle will usually be between days 8 and 15 of the cycle.

Therefore, there are about 6 fertile days: five days before ovulation and the day you ovulate.

The ovum can survive 12 to 18 hours after being released. Then, it will die, marking the end of the fertile days for a woman with a regular menstrual cycle. However, not all women have a regular cycle that lasts the same amount of days month to month.

What if You Have an Irregular Cycle?

It’s more difficult to calculate the fertile period for women with irregular cycles. Therefore, even with these estimates, women may still fail at getting pregnant or prevent pregnancy. If you get your period at different times during your cycle, it’s easy to make a mistake when tracking your days.

However, if you want to calculate the fertile days in an irregular menstrual cycle, this is what you should do. First, you have to record your menstrual cycle for at least six months. For example, one month may last 22 days, another 27, another 31 and so on. After taking these notes, you’ll need to use a formula:

• Subtract 18 days from the shorter cycle and 11 days from the longest menstrual cycle. For example: If the shortest cycle was 22 days and your longest cycle was 31 days, then 22-18=4 and 31-11=20.
• Therefore, your fertile period will fall between days 4 and 20 of the cycle, which is about 17 days. Of course, this estimate is inaccurate and long, but if you’re trying to get pregnant, you should have sex during these days.

When you have an irregular cycle, it’s safest to resort to ovulation tests. These are sold in pharmacies, and you should make sure to follow the instructions very carefully. This is the best way to determine your fertile days, whether you want to prevent pregnancy or want to get pregnant.

Is Knowing Your Fertile Days Helpful?

Yes. Knowing your fertile days is extremely useful.

If, in addition to getting pregnant, you also want to try to influence the sex of your baby, they say that knowing your fertile days can be helpful.

While this theory has not been proven, they say that having sex the exact day of ovulation makes it more likely you’ll have a boy and having sex 3 to 5 days before ovulation makes it more likely you’ll have a girl.

Are There Symptoms that Show You’re on Your Fertile Days?

To calculate your fertile days, you should also know how to recognize the signs that your body is sending you. Among the symptoms that show you’ll be ovulating soon are:

• Increase of liquid in the vagina with a color and consistency very similar to an egg white. The release of the hormone estradiol produces more vaginal fluid than usual. This is the natural lubrication mechanism for the vaginal canal.
• Pimples are the most hated signal of all, but they do indicate that we’re ovulating. In this phase, the skin becomes more oily, even after your teenage years.
• There’s a slight increase in your body temperature, which coincides with the release of the ovules by the follicles. Progesterone is responsible for this increase in body temperature by about one degree.
• Increase sexual libido. This is due to the increase in hormone levels. You feel more attractive and have more sexual desire. Your body becomes your best accomplice.
• There’s an increase in appetite during the ovulation phase. This is because the woman’s body is prepared for a possible pregnancy.
• You might feel a slight pain in the lower part of your belly or cramps that will appear and disappear. This is another indication that you’re ovulating.
• Mood swings. Irritability and emotional instability are also tell-tale signs of ovulation.

Final Considerations

Understanding our bodies well is very helpful when it comes to enjoying our sexuality, whether we want to get pregnant or not. Remember to visit your gynecologist annually to check that everything is in order with your menstrual cycle and reproductive health.

Leading a healthy and balanced life is also essential for increasing your fertility. Avoid stress, relax and enjoy those encounters with your partner no matter if you want to get pregnant or not..