What is the fertile window?

Written by:, PhD, Founder and Inventor of the Proov test — the first and only FDA-cleared test to confirm successful ovulation at home.

Whether you’re actively TTC, working to understand your future fertility, or just dipping your toes in the water, you may have heard the term “fertile window.” Generally speaking, this is the time in your cycle that you can actually get pregnant, so if you’re TTC, this is the time to have sex! 

You may be wondering more, though–like when is the fertile window, how do you find it, and how does it relate to other important parts of the cycle? Read on to learn the answers to all these questions, and more. 

what is the fertile window?

You may be wondering when is the fertile window, how you find it, and how it relates to the other important parts of your cycle. Let's dive in!

What defines the fertile window? 

The time that sex can actually result in conception is defined by two things: sperm survival and egg survival. Ovulation occurs once per cycle, and after ovulation the egg lives for 12-24 hours

Fortunately, you have longer than just that 12-24 hours to try to conceive each cycle, and that’s because healthy sperm can live 3-5 days in the female reproductive tract. What this means is that you could have sex on one day, and if you ovulate up to 5 days after that, there may still be live sperm available to fertilize the egg. 

Putting together the five possible days of sperm survival and the one possible day of egg survival, we get a total of six consecutive days every cycle during which sex can result in conception. That’s the fertile window! 

Of course, because not all sperm will survive as long as five days, all days in the fertile window are not equivalent. While it’s possible to conceive any of those 6 days, it’s more likely at the time right before ovulation, which is often termed “peak day” or “peak fertility.” 

When is the fertile window? 

Because fertility varies throughout the fertile window, it’s important to understand both when the fertile window opens and closes. You already know that the fertile window is the five days before and one day after ovulation, but when is ovulation? 

Your cycle (the time from one period to the next) is divided into two phases, with ovulation at the center. These are called the follicular and luteal phases, based on what’s going on in the ovary at the time. 

The follicular phase begins on day one of your period, often termed “cycle day 1.” During this phase, follicles in the ovary begin growing and maturing to prepare for one of them to release an egg on ovulation day. 

This phase lasts an average of 14-19 days, culminating in ovulation. The last five days of the follicular phase are the first part of the fertile window, meaning the fertile window opens, on average, 9-14 days from the beginning of your period. 

Ovulation begins the luteal phase, or the half of your cycle during which progesterone is secreted and the uterine lining prepares for the implantation of a newly fertilized egg. The first day of the luteal phase includes ovulation day, and is the last day of the fertile window. 

The luteal phase lasts on average 11-13 days, and then your next period will begin if you aren’t pregnant. If your period comes early or your luteal phase is shorter than this, this may be an indicator of something called a luteal phase defect, and it may make getting pregnant more difficult. 

If your period doesn’t come by this time, it may be time to start testing for pregnancy

what is the fertile window?

If your period doesn't come at the end of your luteal phase, then you may want to test for pregnancy!

How do you detect the fertile window? 

So you can see from our above calculations that the fertile window occurs roughly in the middle of your cycle. Especially if you have pretty regular cycles, you may choose to just estimate your fertile window based on your average cycle length. In fact, that’s how most period trackers or ovulation calculators work. 

This doesn’t really work for irregular cycles, though, and doesn’t account for real-time fertility, hormonal changes, or people who just have follicular phases that are shorter or longer than average. Timing intercourse correctly is pretty important for conception, so you don’t necessarily want to just guess and end up missing the fertile window entirely. 

Fortunately, there are many hormonal biomarkers you can use to detect the approximate time of the fertile window! The hormone estrogen rises during the follicular phase and indicates the beginning of the fertile window, and the hormone progesterone rises after successful ovulation and indicates the end of the fertile window.  

There are a variety of different ways you can approximate the beginning and end of the fertile window to try to conceive. You can pick your favorite, or use multiple. Or, you can find the entire fertile window with Proov Complete, which includes several of these methods!

what is the fertile window

Find the beginning of the fertile window: 

  • Cervical mucus: Your cervix secretes cervical mucus, which is a hydrogel containing salts and protein that allows for sperm survival and transport. The consistency changes in response to estrogen rising, and cervical mucus becomes noticeable, slippery, clear, and more fluid. If you notice this change, it’s a sign that your fertile window is opening! You can also detect the fertile window via the changing electrolytes in your cervical  mucus using kegg
  • Estrogen testing: When estrogen rises at a certain rate, it indicates that ovulation is a few days away and the fertile window is opening. You can test urinary E1G (a metabolite of estrogen) with Proov Complete, which detects the full fertile window using this estrogen rise. 

Find the end of the fertile window: 

  • Ovulation tests/ovulation predictor kits: Ovulation tests like Proov Predict detect luteinizing hormone, a hormone that surges 24-36 hours before ovulation. If you get a positive ovulation test, it’s highly likely that you’ll ovulate in the next day or two, and you can consider the fertile window over the day after that. It is possible to ovulate with a negative test, though, and especially if you have certain conditions like PCOS, a positive test doesn’t guarantee ovulation. That’s why it’s important to confirm ovulation too. 
  • Progesterone testing: When the egg is released, progesterone begins to rise to prepare for implantation. Proov Complete also detects PdG, a urinary metabolite of progesterone, and when PdG rises, this indicates that the fertile window is over. 
  • Basal body temperature: When progesterone rises, your body temperature rises too! By taking your basal body temperature (your temperature at rest, right as you wake up at the same time each morning), you can observe a temperature rise of usually at least 0.3ºF within 1-2 days around ovulation. This temperature shift will last until just before your next period. If you observe this temperature rise, you can consider the fertile window over after 3 days. 
  • Cervical mucus: The consistency of cervical mucus can help you find the end of the fertile window too! If you notice several days of slippery, egg white-consistency cervical mucus followed by an abrupt shift to dryness or disappearance of cervical mucus, this is likely the effect of progesterone on your cervix, and the fertile window is likely over. 
BACK TO TOP