Calendar methods get a really bad rap in the fertility awareness communities. A lot of this is for a very good reason. When the calendar rhythm method was discovered in the 1930s, it was revolutionary. However, since then, we have discovered real-time fertility signs such as cervical mucus, basal body temperature, and urinary testing.
To illustrate why calendar-based rules can be both risky and occasionally line up with real-time signs, I charted with three calendar-based methods for opening the fertile window versus a method with real-time fertility signs only (specifically I used the Billings Ovulation method for my real-time method).
In the first line, you will see the Natural Cycles method. This method relies on basal body temperature as its only required real-time sign. Unfortunately, basal body temperature can only tell you when your fertile window closes, not when it opens. Temperature has no predictive qualities for letting you know if you are going to ovulate at a different time than normal.**
With only 3 cycles of my previous data, Natural Cycles gave me until day 8 as safe for this cycle. What Natural Cycles doesn’t know is that my cycles range from 24 to 30 days long over a calendar year. For this reason, it can be quite risky some cycles for me to be allowed safe sex until day 8. The fact that this cycle happened to line up is merely a coincidence! This coincidence can cause a confirmation bias when people use this app and do not get pregnant. If you are seriously avoiding, be wary of any method that doesn’t allow you to crosscheck the opening of your fertile window.
Natural Cycle also closed my fertile window in an incredibly risky manner. It told me I was safe on the morning after my real-time sign of “peak” day. In fertility awareness based methods, “peak” is the highest level of fertility you can get in a cycle. The two days following Peak day also have a significant chance of ovulation occurring. Every time I have used Natural Cycles (here is my previous try with it last year), it gives me a very risky closing to the fertile window.
Here is my full chart from Natural Cycles this time:
Next up on the chart above illustrating my safe days is the Daysy thermometer. Daysy is a thermometer that relies on calendar based rules and potentially earliest temperature shift based rules to open the fertile window. In my two experiences with Daysy, it has been much more conservative than Natural Cycles. Daysy does learn over time, so it is possible I could have a risky day with it in the future, but so far I have not had any risky days with Daysy.
You can see in the image above that Daysy confirmed ovulation last out of all of the methods. I believe this is because my temperature shift was a bit erratic and because the device does not allow the user to mark temperatures questionable (I had two that were marked questionable for my own manual interpretation).
Daysy does not rely on anything except cycle length and temperature shift timing to open the fertile window. For this reason, Daysy can be risky if you ever have a very early ovulation. It can also be risky if the user is not careful about only taking their temperature when it is not disturbed, or if the device misreads a temperature shift. In my experience, Daysy tends to be much more cautious than Natural Cycles.
My third line is a true calendar only method. DOT has over a year of my data; however, the prediction has only given me one extra safe day during my whole use of the app. DOT is entirely based on the calendar method. However, interestingly DOT got higher efficacy than Natural Cycles in their study. Here is my DOT chart below
While I do not recommend the calendar method to most charters, this app can be useful for period prediction or for birth control if you are okay with an unintended pregnancy if you were to suddenly have a longer or shorter cycle. Users for DOT must have no more than 8 days variation in their cycle per calendar year.
In the example above, DOT actually gave me no risky days whatsoever. Again, this is a coincidence that it seemingly lined up with other signs. At any time, cycles can always change.
Finally, my main method is the Billings Ovulation Method. Billings relies only on real-time fertility signs. This means that they reject any calendar-based thinking, including the idea that menstruation is automatically safe. My Billings chart was based on when cervical mucus opened the fertile window (cervical mucus is what helps sperm survive) and when cervical mucus peak rules closed the window (when sperm can no longer access the cervix because ovulation is over and the cervical mucus plug has closed).
I always recommend real-time fertility signs to anyone who wants very high efficacy, the least amount of consecutive abstinence, who may be in regular or irregular cycles, and who want to understand their body and their health on a more deeper level. Here is an example of a Billings method chart. The babies represent possibly fertile days.
I hope this post helps you think critically about whether calendar-based methods for opening the fertile window are right for you!
Here is a breakdown of efficacy for these methods:
Daysy: Claims 99.4% perfect use, but their study was retracted.
Natural Cycles: 98% perfect use, 93% typical use.
DOT app: 99% perfect use, 95% typical use.
Billings Method: 97.8 to 100% perfect use, typical use varies depending on country.
**Some modern fertility awareness methods use the Doering Rule to set the opening of the fertile window. This can be very safe and yield high efficacy. Doering is based on the earliest temperature shift of all time (not just the last year).