How to Know Your Fertile Days Naturally & Precisely

If you’re considering a natural birth control method, the first thing you need to learn is how to know your fertile days.Fertile days, the opposite of your safe days, are the days when you stand a higher chance of getting pregnant.

But to tell safe days apart from fertile days, you first need to understand your ovulation cycle.

The ovulation cycle has 4 phases

The ovulation cycle occurs in every healthy female’s body throughout her adult life, except for when she’s pregnant.

The average cycle lasts from 28 to 31 days. But Many women have irregular cycles, or simply cycles that are longer or shorter than average.

But all cycles go through the 4 following phases:

fertile days

Follicular phase (Days 6-11)

  • At this phase, after each period, your body releases a new follicle (an immature egg cell) 
  • While this follicle matures, your cycle is in its follicular phase.
  • Once the follicle is fully mature, it becomes an egg. This prompts ovulation around the 14th day.

Fertile window (Days 12-16)

Luteal phase (Days 17-28)

  • After ovulation is finished, the egg turns into a corpus luteum (“yellow body”) and the cycle enters its luteal phase (before the period).
  • Hormones cause the endometrium (uterine wall) to thicken.
  • If the egg was not fertilized it will dissolve, causing hormone levels to drop.

Period (Days 1-5)

  1. The endometrium (uterine wall) sheds and a new period occurs.

Warning! Understand the fertile window

  • Ovulation only lasts 12 to 24 hours (around the 14th day) but sperm can survive within the body up to 5 days. This means the days leading up to ovulation, at the end of your follicular phase, are your most fertile days. We call this the fertile window.
  • Ovulation cycles usually last between 28 to 31 days. This means ovulation will not occur on the 14th day for every woman.
  • Many women have irregular cycles. So a calendar alone is not a reliable method to detect your fertile days.

How to use the ovulation cycle to know your fertile days

Each stage in the ovulation cycle is associated with different hormone levels. This leads to changes in basal temperature and cervical mucus at each phase of the ovulation cycle.

Monitoring basal temperature and the cervical mucus throughout the cycle is key to spot fertile days. Or, in other words, they work as natural fertility awareness methods.

Let’s focus on these 2 methods to explain how you can know your fertility days, the easy way.

The basal body temperature method

During the follicular phase, after the period, basal body temperature is slightly lower. It then rises 0.3ºC – 0.6ºC after ovulation and stays that way during the luteal phase. Since these changes are so subtle, it’s best to use a fertility thermometer to track them precisely and ahead of time.

How to take basal body temperature to track fertile days

  • After waking, while laying in bed
  • Check every day, at the same time
  • Use a fertility thermometer (accuracy for each 0.01ºC or 0.10ºF)

For more info read our article: How your Fertility Monitor knows when you’re fertile?


Sleep changes, jetlag, heavy drinking, and illness can also spark changes in basal body temperature. If you are feeling ill or are traveling, those readings may be unreliable.

Fertile cervical mucus method

Another effective way to spot your fertile days is by monitoring your cervical mucus. This is also known as the ovulation method or Billings method. The mucus changes throughout the ovulation cycle in color and consistency. This method can be Complementary to a fertility monitor.

How to use your cervical mucus to know your fertile days

  • After your period, you may not notice any discharge. These are “dry days” and are usually safe days
  • Before ovulation, your mucus will be white to yellow and sticky. You’ll notice it at the opening of your vagina and maybe on your underwear
  • Before ovulation, the mucus is abundant, clear and will feel slippery, like raw egg whites. These are your fertile days (unsafe)
  • After ovulation, your mucus will get sticky again and the color will again revert to white or pale yellow. Once you notice you’re becoming dryer, you’re no longer on your fertile days.

How to check your cervical mucus

  • Check your mucus every day and record what you notice
  • Check your cervical mucus before urination, using white toilet paper or tissue
  • Look at any discharge that might appear on your underwear
  • If you think you’re close to ovulation, pull your mucus between your thumb and your index finger. Your mucus should be abundant, slippery and will stretch between your fingers.


  • if you notice your mucus has a darker yellow, green or even brown color, you might have a vaginal infection. Therefore you cannot rely on this method to spot your safe days.
  • if your mucus turns white with a thick consistency, like cottage cheese, you may have a fungal infection. You cannot rely on mucus examination to spot your safe days until the infection disappears.


In this article, we covered how you can tell your fertile days from your safe days. We learned that your fertile days are the days that you’re ovulating but also the days before ovulation (during the end of the Follicular phase).

The best natural and accurate way to detect those days is using a fertility monitor that measures our basal body and tells us when we have a safe day or a fertile day.

More about safe days

  • How to know your fertile days naturally & precisely.

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