How to Track Your Most Fertile Days

by | Jun 11, 2018

If you’re actively trying to get pregnant, you probably want to do it in the most efficient way possible. You want to make sure you’re having enough sex at the right time, but between the thermometers, smiley faces and the calendar method, which assumes every woman ovulates on the same day of her cycle, you could be feeling frustrated.

It is possible to determine your most fertile days; you just have to separate the outdated information from the most recent scientific findings.

First, The Outdated Information

For years, clinical guidelines said that women’s periods lasted for 28 days on average and that women ovulated on the 14th day of their cycle (the first day of your cycle is the first day of your period).  Common wisdom held that women were fertile for six days every month: the day of ovulation and the five days leading up to it, meaning most women were fertile between days nine and 14 (although clinical guidelines state women are traditionally fertile between days 10 and 17 of their cycle).

In addition to this, women are still encouraged to buy a special thermometer with which to take their basal body temperature (i.e. their temperature first thing in the morning before they get up).  A rise in basal body temperature (BBT) of 0.5 degrees Fahrenheit is supposed to indicate ovulation has occurred.

The Most Recent Studies

But recent studies have shown the above information is outdated.  A study published in November 2000 found that only 30 percent of the women studied had a fertile window contained completely within the 10- to 17-day time period outlined in the clinical guidelines.  Most women reached their fertile window earlier and some much later.  Of the women who had very regular 28-day cycles, only 10 percent of them ovulated consistently on day 14.

Recent research also shows that your body’s BBT may not be as accurate a reflection of your cycle as previously thought.  During ovulation, your body releases the hormone progesterone, which can slightly increase your BBT.  However, it takes about 24 to 36 hours for the rise in BBT to occur, so by the time your thermometer shows an increase, your fertile window will be closing quickly.

There are also situations like drinking alcohol the night before and getting up in the middle of the night to use the restroom that can affect your BBT reading.

So how do you know when your fertile window is? There are a couple ways to test.

Testing Your Hormone Levels

Estrogen, follicle stimulating hormone (FSH), luteinizing hormone (LH) and progesterone are all hormones your body secretes at different parts of the ovulation process.  FSH is responsible for the growth and maturation of your eggs, while estrogen and LH stimulate ovulation.  The estrogen, FSH and LH levels detected in your urine will rise close to your ovulation day, with estrogen and FSH peaking at ovulation.  Your LH level could also peak at ovulation, or it could peak afterward, depending on your body. In a study published in 2003, researchers found that 77 percent of the menstrual cycles studied saw an LH rise right before ovulation, while 67 percent saw an LH peak either a day before ovulation or a day after.

Given this information, you might think: why bother with the LH test, why not just test for peak FSH and estrogen levels?  Well, unfortunately, testing your estrogen and FSH levels is expensive (and FSH doesn’t trigger ovulation) so most ovulation predictors on the market test the LH concentration in your urine. But that doesn’t mean you can’t get valuable information from testing your LH levels.

A study published in 2017 found that once the LH concentration in the participants’ urine reached 25-30 mIU/ml, there was a high probability they would ovulate within the next 24 hours.  Researchers also found that using LH testing alone indicated false fertile windows in 1/3 of the cycles.  So they recommended combining LH testing with the more low-tech method of checking your cervical mucus.

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Checking Cervical Mucus

Your cervical mucus (CM) is fluid secreted by the cervix in order to protect the sperm and help them swim through an otherwise hostile environment so they can get to, and fertilize the egg.  As your estrogen levels rise, preparing your body for ovulation, your cervix starts to secrete more CM.

As you enter your fertile period, the mucus will also change in texture, moving from a yellowy or white, cloudy and somewhat sticky substance, to something that resembles the color and texture of egg whites.  After you’ve ovulated, your mucus will decrease in quantity and become creamlike in appearance.

To check your CM, doctors recommend washing and drying your hands and then sticking your index or middle finger into your vagina, getting as close to your cervix as possible, or wiping your vagina front to back.

Dr. Erin Niemasik, an OB/GYN at Kaiser Permanente, doesn’t recommend relying solely on CM changes to determine your fertile window.  “I usually don’t recommend this method as the subtle changes in the cervical mucus can be difficult for women to interpret.”

And Crystal Meredith echoed this sentiment saying, “Before we tried IVF, I was checking my cervical mucus every day but it always looked the same to me.  It was so frustrating because I felt like I was supposed to be seeing something that I wasn’t.”

But researchers in the 2017 study found that when combined with LH testing, monitoring the changes in the CM and comparing them the concentration of LH in the woman’s urine, led to a more accurate prediction of when she would ovulate.

The bottom line, researchers recommend tracking your data every day for at least three months before trying to determine your fertile window. And even then, they caution that window could move from month to month so you’ll need to continue testing to make sure you’re trying at the right time.

The contents of this blog were independently prepared, and are for informational purposes only. The opinions expressed herein are those of the author and are not necessarily indicative of the views of any other party. Individual results may vary.



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Ready to easily, precisely, and automatically track your ovulation cycles? Let Mira take the guesswork out of getting pregnant, so you know exactly when to conceive.