There is a lot of conflicting information surrounding the fertile window and how to measure when you can get pregnant (and when you can’t!) each cycle. How are you supposed to know which measures of ovulation to take, and which to leave behind? Today, I’m going to debunk the fertile window and cover the two most reliable ways to measure your fertile window, plus a couple runner ups.
What is the Fertile Window?
If you are naturally cycling (that is, you’re not on hormonal birth control, in menopause, or breastfeeding) there are four main hormones at play each cycle. These are estrogen, progesterone, luteinising hormone (LH) and follicle stimulating hormone (FSH).
At the beginning of your cycle (day 1 being the first day of full bleeding) the hormone FSH is recruiting a follicle, or immature egg to develop. Estrogen slowly rises and begins maturing the follicle. Once it is ready to be released, the hormone LH peaks, triggering ovulation. Following ovulation, estrogen is replaced by the hormone progesterone as the dominant hormone, which maintains the uterine lining until your next period.
Ovulation, or the release of an egg, is the main event of the cycle. Ovulation creates the domino effect that leads to your next period, and your next cycle. The fertile window is the length of time each month where pregnancy is possible. This information is key for folks who are wanting to both avoid pregnancy with FAM and get pregnant.
When we’re measuring the fertile window, we’re taking into account both the length of time that an egg survives, and the length of time that sperm can survive to fertilize an egg. Depending on how long a woman’s cycle is and her hormonal activity prior to ovulation, this window of time can vary. In general, though, a woman can only conceive 5-7 days per cycle.
Key Marker 1: Basal Body Temperature (BBT)
The first way to measure ovulation is by tracking your basal body temperature (or BBT, for short). Your BBT is the lowest body temperature attained during rest. After you release an egg, progesterone raises your body temperature very slightly. You can measure that slight raise in temperature with basal body temperature as an important ovulation symptom.
Source: Apple Seed Fertility
BBT will only tell you ovulation has happened AFTER the fact! It is a great objective measure of ovulation, because you can actually see on your chart that it has taken place. Some things can skew your temperature readings, like drinking alcohol the night before, sleeping in a warmer or colder room than normal, or taking your temperature earlier or later than normal (unless you have Tempdrop, in which case you don’t need to worry about those things impacting your temps!)
Key Marker 2: Cervical Mucus
It’s unfortunate, but cervical mucus gets a gross reputation. This means we haven’t been educated or made aware of the importance of cervical mucus for both avoiding pregnancy and getting pregnant. I only learned about the importance of cervical mucus when I first learned how to use FAM. Up until that point, I thought it was a mildly gross nuisance. Cervical mucus is in fact very cool and really quite essential for getting pregnant!
Cervical mucus is a hydrogel secreted by crypts in your cervix. These crypts, or tiny ridges, are influenced by the presence of estrogen and progesterone. Cervical mucus has the important job of keeping sperm alive and nourished, keeping infections out of your uterus, and helping only the highest quality sperm reach a potential egg. Without cervical mucus, sperm would die very quickly in the acidic vaginal environment.
The onset of cervical mucus after menstruation will let you know that your body is gearing up to ovulate, and tells you that your fertile window has opened. This is because with the presence of cervical mucus, sperm stay nourished and alive for up to 5 days! Another important thing to note with cervical mucus is that when you experience “peak” type mucus, it is around your most likely time for intercourse leading to conception. You can pay attention to cervical mucus in three categories.
Dry Cervical Mucus
These are days where you don’t experience any cervical mucus. Keep in mind that you may always notice some moisture as your vagina is a mucus membrane.
Non-Peak Cervical Mucus
Days where you notice opaque, creamy, lotiony, not-stretchy cervical mucus, with a moist sensation when you wipe.
Peak Cervical Mucus
Days where you notice stretchy (more than an inch), clear or partly-clear, or watery cervical mucus, with a lubricative sensation when you wipe.
Runner Up: Cervical Position
Just like cervical mucus is influenced by your cyclical hormones, so is the position of your cervix. Your cervix is the “gateway” between your vagina and your uterus, and you can find it by sticking a finger in your vaginal opening like you would put a tampon in, and feeling for a small nub that has a divet - kind of like a tiny cute donut. The divet is your cervical opening, and may be larger or smaller depending on if you’ve delivered a baby.
When estrogen is low, before and after your fertile window, your cervix will be firm, low, tilted and closed. When estrogen is high, around ovulation, your cervix will be soft, high and open. Checking your cervical position each day will give you another optional fertility sign letting you know how close ovulation is!
Runner Up: Luteinizing Hormone (LH) Tests and Ovulation Prediction Kits (OPKs)
LH Tests are urine tests that measure the levels of LH in your urine. If you remember from the beginning of this post, LH is responsible for triggering the release of an egg at ovulation. You will experience an LH surge approximately 24-36 hours prior to ovulation, and this is what you are trying to catch when testing with LH tests.
How to Test Luteinizing Hormone
LH tests will usually come individually wrapped, or you can buy a more expensive Ovulation Predictor Kit (OPK) that does the readings for you. When you start noticing the shift to moist, non-peak cervical mucus after ovulation, that is the time to start testing LH. Test at the same time each day, by peeing straight on the stick or dipping it into a cup of your urine. A positive LH test looks like a dark line that matches the “control” line.
And that’s it! The four signs I recommend for measuring your fertility! Now you know why you can rely on yourself to measure your fertile window, rather than an app or device.
If you'd like to learn more about FABM, including how to chart your cycle and identify your fertility window, download Tempdrop's free Introduction to Fertility Awareness, a beautifully-designed guide written by seven certified fertility educators (including Nathalie!).
Nathalie Daudet is a social worker and FEMM instructor based in Winnipeg, Canada. She discovered fertility awareness after searching high and low for a non-hormonal method of birth control. After learning the magic of fertility awareness and the gift of body literacy, she decided to pursue formal fertility awareness training and share the knowledge of fertility awareness with women looking for a natural birth control option. Fertility Awareness Project is the hub for Nathalie’s FEMM classes in both group and individual formats, online and in person in Winnipeg. If you love this post and would like to thank Nathalie, shop Tempdrop with Nathalie's unique referral link.