What is a Fertile Window?

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Your fertile window is the 6 days in a menstrual cycle that you have a high chance of conceiving through having sex. It usually includes the day of ovulation and five days leading to ovulation. Once an egg is released into the fallopian tube, it has only up to 24 hours of lifespan. Fortunately, sperm may survive in women’s reproductive system up to 5 days during the fertile window so fertilization can occur. If you are trying to conceive, these are the days you don’t want to miss. The closer to ovulation that the sperm is there, the more likely conceiving could occur. So, sex close to ovulation is the best to make a baby.


When is my fertile window?

It is generally believed ovulation occurs about halfway through a cycle – 14 days before the onset of your next period. See Mira’s ovulation calculator about how to calculate your fertile window. However, ovulation timing varies greatly from woman to woman, and cycle to cycle, depending on the length of a cycle and consistency of your pattern. Stress, lifestyle, malnutrition can influence the cycle regularity too. Since there are so many factors out there, looking at the period along is often not enough to identify ovulation.

The number of days sperm can survive within the female reproductive tract depends on if the environment is right, including the vagina, uterus, and fallopian tubes. During the fertile window, the fertile cervical mucus helps the sperm to survive and swim to the fallopian tube, where it can live up to 5 days to fertilize an egg. Without the fertile cervical mucus, the sperm can’t survive longer than a few hours.

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How do I predict my fertile window?

Ovulation Hormone

Estrogen level often starts to rise about 5 days before ovulation. A high level of Estrogen triggers the release of Luteinizing Hormone (LH). This is what starts the process of ovulation, which usually happens 12-24 hours after the LH surge.

A mature egg is released from the ovary when you ovulate. The egg travels down the fallopian tube and has 12-24 hours to be fertilized by sperm.

The LH test is widely used to predict ovulation. You are at your peak fertility when LH raises. The estrogen test gives you extra 4-5 days of the predicted fertile window. You will want to capture both if you really want to get pregnant.

Ovulation prediction kits (OPK) are often used to test LH. Because every woman is different, and hormone level can vary from cycle to cycle, OPK sometimes misses your peak when judging ovulation by a fixed hormone threshold. If you have PCOS, irregular cycle, or approaching menopause, OPK test can be difficult, unreliable, and frustrating. Check out how Mira’s hormone test self-adjusts to your personal condition so you can’t miss a peak.


Basal body temperature

Your basal body temperature (BBT) is your temperature at rest. It increases about 0.5 Fahrenheit 12 hours after you ovulate. BBT tells you that you have ovulated but can’t predict your ovulation. Your most fertile days are the two or three days before your BBT rises.

You will need to take your BBT immediately after waking, while still in bed, after at least 4 hours of sleep without interruption. You will want to note and void days affected by environmental factors, such as sickness, fever, medication, air condition, or heater, which can cause changes in your BBT. It usually takes several cycles of consistent effort to understand your BBT.


Cervical mucus

When you approach ovulation, your Estrogen level rises, which causes the cervix to produce fertile cervical mucus, which looks like egg white. It is clear and stretchy. It protects sperm and helps them to swim towards the egg.

The best way to learn how to observe your mucus is to observe a sample daily. Clean your hand and insert a finger into your vagina and observe the change of texture of your mucus. The days with the highest amount of mucus production and egg white textures are often your fertile window.

It usually takes several months to learn how to identify fertile cervical mucus. This is also called the fertility awareness method.


How does Mira predict my fertile window?

Mira measures both Estrogen and LH hormones, which gives you the complete fertile window. Mira’s AI learns an individual’s hormone pattern, meaning that you don’t have to worry about missed peak due to personal variability. Simply pee on the Mira test wands, the analyzer does all the rest of the job. Just like a lab test, Mira measures your actual hormone concentration, and the App tells you when you are fertile, and how fertile you are.




  1. Megan

    What time of the day do you test your urine? I have been told the first owe in the am and mid day after restricting all flyids for 2 hours..

    • Mira Fertility

      Hi Megan, thanks for your question! Morning urine is often recommended because fluid intake is limited and it is usually easier to manage. However, LH surge in urine often occurs in the afternoon. That’s why you will want to test again during the day after restricting all fluid for 2 hours. You only need to do so on your LH surge day. If you use Mira, the App automatically calculates to you and tells you exactly when you will need to test.


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