What is the Fertile Window?

By
Kate Causbie
/
/
January 26, 2018

The purpose of Fertility Awareness Based Methods is to identify fertile days, or the fertile window, in a given cycle. The fertile window is the time during which FABMs indicate conception is possible, and thus unprotected sex is not safe without risking conception. Using FABMs correctly means identifying fertile days accurately through the rules of your method of choice. But do you know how to determine if a day is fertile or not?

In the Symptothermal Method, the fertile window is identified by combining the lifespan of sperm with the lifespan of the egg:

      Once we know sperm can survive (to meet an egg), the fertile window opens.
      Once we know the egg or eggs are gone (and thus no sperm can meet an egg for the rest of the cycle), the fertile window closes.

Opening the Fertile Window: Sperm-Friendliness

The fertile window is opened by the presence of fertile cervical fluid. Fertile cervical fluid matches the pH of sperm, and thus keeps sperm alive in order to meet an egg. Sperm can typically live 3-5 days in fertile cervical fluid. The catch, when identifying the fertile window, is that we don’t know when in the future ovulation is going to occur (that is, when an egg will show up to meet those sperm), so we can’t simply know what day to count back 3-5 days from. To cover all possibilities, as soon as we observe fertile cervical fluid we must consider the fertile window open... the egg could be released in 3 days, or in 10.

Closing the Fertile Window: The Egg’s Lifespan

The hormonal event where an egg is released to meet said sperm is called ovulation. Ovulation only occurs once each cycle. A single ovulation event might release two eggs, but it is still considered only one ovulation event, and the eggs are released during the same 24 hour window.

It might be helpful to think of ovulation as similar to the season when fruit is ripe on a trees (other than the fact that ovulation is a 24hr window and ripe fruit is a window of a few months). But in both cases, it’s not a matter of an individual fruit ripening. It’s a chemical process that occurs in the plant all at once, and that may or may not affect multiple pieces of fruit. The season of fruit ripeness starts, all the fruit ripens and eventually falls from the tree, and then the season ends and the fruit rots back into earth and is no longer available to us. The process of ripening doesn’t begin, ripen one fruit, stop and go back to flowering, and then ripen another. Once it’s happening, it’s happening… it could produce one fruit, it could produce 30. And once it’s done, it’s done.

Back to humans. Any and all eggs will be released within one 24 hour window. After the moment(s) the egg or eggs are released during ovulation, each has its own ticker of between 12 and 24 hours until it dissolves. It must meet a sperm during that window in order for conception to occur. For TTA purposes, the symptothermal method covers its bases and assumes that any potential multiple eggs were released as far apart as possible: at either ends of that 24 hour window of ovulation. Since they each get up to 24 hours to find a sperm before they disintegrate, we assume that we must wait a full 48 hours to be sure that both eggs have indeed passed by. Furthermore, assuming we don’t constantly take an ultrasound of our ovaries, we have to confirm that ovulation did actually occur through the Peak Day and Temp Shift Rules.

The Peak Day Rule says that we must mark our last day of “peak type” (eggwhite or watery) cervical fluid and then count four days after that day without peak type fluid returning. The evening of the 4th day that happens, the Peak Day Rule is met.

The Temp Shift Rule says that we need to wait for three consecutive basal body temperatures that fall above our chart’s coverline. Then the Temp Shift Rule is met.

When both of these rules are met, the fertile window is considered closed, the egg is gone, and those TTA users are considered safe for unprotected sex. These rules cover at least 48 hours after the eggs are released, and usually longer.

A common challenge to the science of the fertile window is, “but can’t you ovulate twice in one cycle?!”

Fear not. The answer is: a solid nope! If you ovulate, you’ll ovulate once at some point during the fertile window (always... even if the fertile window is misinterpreted by a user, the nature of ovulation occurring is that the day it occurs is within the fertile window) in a given cycle, and then your body is on track to bounce through the luteal phase and have your period. The Peak Day Rule and Temp Shift Rule are the only fertility signs (not counting interventions like progesterone tests and ultrasounds) that confirm ovulation actually occurred. People who suspect they ovulated twice in one cycle are actually noticing or referring to one of two things:

a) The release of two eggs. This is, as above, still only ovulating once. The eggs will be released during the same 24 hour ovulation event, and each be considered to have 24 hours of lifetime. Ovulation doesn’t release an egg, stop, and then start back up again. But it can start, release an egg, release another, and then stop. (More on this above, with the fruit metaphor!) We can’t be certain exactly when ovulation begins, but once the Peak Day and Temp Shift Rules are met, we can be confident that it is completed.

b) Double, or even triple, attempts to ovulate. Many people “feel” their bodies preparing to ovulate: they get mid-cycle/ovulation pain or sensations, notice an increased sex drive, or observe fertile cervical fluid. It’s true these are all signs that connect to ovulation, but they connect with the body expecting or preparing to ovulate, not actually having completed it. The body can prepare multiple times to ovulate in a cycle, but we can’t be sure it did until the Peak Day and Temp Shift Rules are met. A few attempts at ovulation in one cycle is typical if you get sick, travel, sleep less, or experience other changes or stress during your follicular phase. In one preliminary study at the University of Saskatchewan ("A new model for ovarian follicular development during the human menstrual cycle", out of fifty women with regular menstrual cycles, 68% of cycles took two “waves” of ovulation preparation before ovulation occurred, and another 32% took three “waves.” This means one or two days that looked like Peak Days (but of course the temp +3 rule wasn’t met until ovulation actually occurred) before the “real” Peak Day that led to both post-ovulation rules being met.

The Symptothermal Method takes this into account: this is why the fertile window doesn’t close until ovulation is totally confirmed.

Identifying fertile days is the foundation of Fertility Awareness Based Methods, and understanding exactly how and why we identify them is key to using the methods correctly!

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