Learn How to Calculate the Fertile Period

For women who are trying to conceive, the most important thing to become familiar with is your menstrual cycle. This is because without understanding your cycle, you won’t know when ovulation occurs, which is also when you are most fertile.

Understanding the menstrual cycle may also help women who want to avoid conception by only having sex during the safe period outside the fertile window. In both conception and non-conception, timing of sexual contact is the key. 

This article will discuss the menstrual cycle, the concepts of ovulation and the fertile window, and the methods for calculating the fertile period. It will also cover apps you can use for fertile period calculation, as well as tips on how to maximize fertility to increase the chance of conception.

  • Understanding the Menstrual Cycle
  • Characteristics of a Normal Menstrual Cycle
  • Causes of Irregularities in the Menstrual Cycle
  • Learning About the Fertile Period
  • Calculating Your Fertile Window
  • Fertility Apps You Can Use
Learn How to Calculate the Fertile Period
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Understanding the Menstrual Cycle

Before we delve into calculating the fertile period, it’s important to first understand the menstrual cycle to grasp the concept of ovulation and the fertile and safe periods.

Learn How to Calculate the Fertile Period
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According to the Mayo Clinic, the menstrual cycle refers to the monthly series of changes that the female body undergoes to prepare for the possibility of pregnancy

Ovulation is the part where one of the ovaries releases an egg. During this time, the uterus is prepared for pregnancy via hormonal changes. 

If the egg isn’t fertilized during ovulation, the uterus’ lining sheds through the vagina, the process of which is called a menstrual period.

A Deeper Look at Ovulation

Once an ovary releases an egg, it moves to the fallopian tube and stays there for at least 24 hours. If a sperm is able to reach the egg and successfully fertilize it, this is when pregnancy occurs.

However, if there was no sexual contact during ovulation or no sperm reaches the egg during this waiting period, the egg will move down into the uterus and break down.

This broken-down egg will be expelled outside of the woman’s body during the next menstrual period.

Characteristics of a Normal Menstrual Cycle

The menstrual cycle of an average woman lasts between 28 to 32 days. However, some women have much shorter or much longer cycles compared to others.

Learn How to Calculate the Fertile Period
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The first day of the menstrual period is considered the first day of the cycle. Menstrual flow may last from anywhere between two to seven days.

Long cycles are common for the first few years of a woman’s menstruation but as you age, it tends to become shorter and more regular.

Tracking Your Menstrual Cycle

Since the menstrual cycle varies from woman to woman, the only way to track what your normal cycle is would be to record it on a calendar. Keep track of the start date for several months to find out your periods’ regularity.

You can also take note of your cycle’s end date to determine how long your period lasts. You should also keep track of your flow by recording its heaviness, including how often you need to change sanitary protection.

Also track if you pass any blood clots, experience any abnormal bleeding, and how much pain you feel when you get your menstrual period. Changes in mood or behavior may also be recorded.

Causes of Irregularities in the Menstrual Cycle

There are various reasons for why irregular cycles may occur. The most obvious reason for a missed period is pregnancy. Breastfeeding after giving birth may also cause delays to the return of menstrual periods.

Learn How to Calculate the Fertile Period
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Menstruation may also be disrupted by eating disorders such as anorexia nervosa, extreme weight loss, or excessive physical activity. Meanwhile, prolonged periods and heavy menstruation may be caused by uterine fibroids.

Irregular periods are also common in women with polycystic ovary syndrome (PCOS) while women who have premature ovarian failure may experience irregular or occasional periods for years.

How to Prevent these Irregularities

In some cases, menstrual irregularities may be regulated using birth control pills. In cases of underlying causes, such as eating disorders and extreme weight loss, professional treatment is required. 

If the reason for your irregular menstrual cycle is excessive physical exercise, try to moderate these physical activities and observe if your menstruation returns to normal.

However, there are cases where menstrual irregularities may not be prevented such as the case of PCOS. Don’t forget to consult your physician if you observe any irregularities for long periods of time.

Learning about the Fertile Period

The American College of Obstetricians and Gynecologists says that ovulation occurs about 14 days prior to the expected date of the next menstrual period if a woman has a monthly cycle of 28 days.

Learn How to Calculate the Fertile Period
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As such, most women ovulate between days 11 to 21 of their menstrual cycle. However, ovulation does not always occur on the same day every cycle and may vary by a day or more before or after the expected date.

The part of the cycle where ovulation occurs is oftentimes referred to as the “fertile window” because this is the period when a woman’s chance to become pregnant is at its highest level.

More about the Fertile Window

If a woman ovulates on day 14, she can conceive on that day or within 24 hours since ovulation started. However, the fertile window actually began several days before ovulation.

This is because a man’s sperm can survive for up to five days inside a woman’s body. So, even if she did not have sexual contact on day 14 or 15, she could still possibly get pregnant if she had unprotected sex between day 9 to day 13.

A study from the journal Human Reproduction revealed the following probabilities of being within the fertile window – 2% at day 4, 58% at day 12, and 5% at day 21 of the cycle.

Calculating Your Fertile Window

In order to calculate your fertile window, you must be able to identify what day your ovulation occurs. To do this, you first need to determine the length of your menstrual cycle.

Learn How to Calculate the Fertile Period
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If you have a regular cycle, note how many days it has been since your first day of bleeding in your last period and subtract 14 days from the expected end of your current cycle. 

This will be your ovulation date. Your fertile window will be from day 9 to day 15. 

If you are planning to conceive, you should have sexual contact within this period. If not, you should avoid having unprotected sex during this time.

Signs that You Are Ovulating

Since ovulation may vary from month to month, keeping track of signs of ovulation may help you in more precisely pinpoint your ovulation date.

The most common signs of ovulation among women include mild cramping in the lower abdomen, and the presence of vaginal discharge that is wetter, clearer, and more slippery, similar to egg white.

Women in their ovulation may also experience a small increase in their basal body temperature and will have a higher sex drive. An ovulation predictor kit or fertility monitor may be used as an alternative in determining ovulation.

Fertility Apps You Can Use

Because we’re now living in the age of mobile devices, there are various mobile apps that you can use to track your menstrual cycle and ovulation. One of the best that you will find is the Clue app from BioWink.

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The Clue Period & Cycle Tracker app is available on both the Google Play Store and the Apple App Store. It offers the most comprehensive tracking tools and uses science-driven algorithms. 

Using the app, you will need to enter in different details, such as cravings, mood swings, energy levels, and signs of bleeding or spotting. 

The basic version of the app is free but the paid version costs about $5 a month or $30 yearly.

Other Useful Fertility Apps

I recommend the Clue app, but you may consider using some other fertility apps available in the market, such as the Ovia Fertility app from Ovia Health and the Glow Ovulation & Period Tracker from Glow Inc.

The Ovia Fertility app is an affordable app that offers fertility window predictions using period and symptom charts. You can download it from the Google Play Store and the Apple App Store.

Meanwhile, the Glow app provides a versatile suite of tools that can help women track multiple stages of their cycles. Download it from both the Google Play Store and the Apple App Store.


Understanding your personal menstrual cycle and determining your fertility window each month are great ways to keep track of your body, regardless of whether you want to conceive or are avoiding getting pregnant. There is no app greater than knowing how your own body works.

Now that you’ve learned about the menstrual cycle, fertile window, and how to calculate it, you should be able to have more anticipation of the monthly changes that you’ll go through. You may also be able to avoid health problems by consulting your doctor for any irregularity in your cycle.