How to Calculate Your Fertile Window

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How to Calculate Your Fertile Window

If you are trying to get pregnant, finding the fertile window is everything. Having sex within the most fertile days significantly increases the chance of getting pregnant. Our calculator at the bottom of this article finds your fertile window and helps to get prepared in advance, and so you know the best time to try.

What is a fertile window?

Your fertile window are the most fertile days of a menstrual cycle. It is the few days that you are likely to get pregnant if having sex. The chance of conceiving outside the fertile window is not zero, but it is very small. So, if you really want to get pregnant, don’t miss the fertile window of every cycle!

The fertile window are the few days around your ovulation. Usually, it means 4 days leading to ovulation, until 24 hours after ovulation. Ovulation happens when the egg is released from the ovary, and it is ready to be fertilized by a sperm if they meet each other at this time. Egg has short life span. It only lives up to 24 hours after the release. Therefore, sperm must meet the egg before degradation happens. That is why there is usually only one fertile day after ovulation.

The good news is sperms could live up to 4 days within women’s mucus, given its good quality. So, up to 4 days leading to the ovulation, is generally calculated as part of a fertile window.

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When am I most fertile?

Days within a fertile window don’t have equal fertility level. Your most fertile days are the day leading to ovulation and the ovulation day. These 2 days are called peak fertility. Your chance of pregnancy is zero beyond 24 hours after ovulation as the egg degrades, and your fertility level gradually increases as you approach the ovulation day.

How does conception happen?

Conception happens when a sperm penetrates the outer layer of an egg, which will then divide and form an embryo. Good sperm and egg quality, as well as the best timing, are must haves to ensure fast conception and healthy baby! The fertilized egg moves down the fallopian tube to the uterus, within 4-6 days after fertilization. It then implants onto the uterus lining and grows there. You should take your pregnancy test two weeks after ovulation.

Calculate your fertile window

You need to calculate when you ovulate, to determine a fertile window. On average, women ovulate 14 days before the starting day of the next period, which is also the starting day of her next cycle. To calculate this, you should note the starting date of your last period, which is considered as the beginning of your last cycle, and your average cycle length, which means how many days have passed between the starting date of your last period and the starting date of the next. Let’s follow the steps below:

  • Note the starting date of your last period
  • Calculate your average cycle length
  • Ovulation day = Starting date of your last period + average cycle length – 14 days
  • Fertile window = 4 days leading to the ovulation day and ovulation day itself

Calculate ovulation

First day of your last period

Length of your cycle

Limitations of the calendar method

Calculating fertile window by counting calendar days is called the calendar method. However, even for women with regular cycle, their timing varies hugely. We may not always have the same cycle length. We may not always ovulate 14 days before the next period. It is no surprise that calendar method usually takes 6 months or more of continuous logging, to get an accurate prediction. To know exactly what’s happening with your body, Mira measures the actual fertility hormones with a urine test, just like brining the lab test to your home. Mira tracks fertility hormones continuously, and the Mira App tells you what to do to either try for or avoid a baby.


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