Why You Should Be Cautious About Calendar-Based Methods (Daysy, Natural Cycles, DOT)

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:

Natural Cycles app

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.

Daysy Chart

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.

DOT the app

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).

Why I Do Not Suggest Using Natural Cycles

Natural Cycles costs around $10 a month, or $80 a year. The app claims to be able to tell you when you are and when you aren’t fertile by using basal body temperature (ovulation strips are optional).

This claim is not true. Natural Cycles only has a 93% typical use efficacy rating. After my experience, it seems even lower than that. I used Natural Cycles for 3 cycles and compared it with the sympto-thermal (STM) method that I use. The method I use is 98.2% effective with typical use.

As a long term charter, I use a doering rule (a rule to limit dry days). My last safe day for unprotected sex is day 5 of my cycle (usually the last day of my period) because of this rule. This rule is included in the high efficacy rates of the sympto-thermal method.

The charts that follow show when my method said I was safe versus when Natural cycles told me I was.

I have compiled 3 of my charts for comparison. During more than one cycle, Natural Cycles told me I could have unprotected sex on the day near my PEAK fertility. Peak day is the most likely day of ovulation, while not always the exact day of ovulation. Suffice it to say, the app told me that I could have unprotected sex on a day when pregnancy was still possible.

September Cycle (First with Natural Cycles)

On my very first cycle, without any knowledge of whether I had ovulated the cycle before, Natural Cycles gave me clearance to have unprotected sex during my period.

This is totally wrong. Without having confirmed ovulation the previous cycle, there is no way to know if this bleeding is safe or not.

The next BIG issue is that it told me I could have unprotected sex on the second day of my temperature shift (CD17). There is a huge chance that my egg could have still been viable and hanging at this point.

With the symptothermal method, you CANNOT confirm ovulation before 3 high temperatures.

The chart below has my actual safe days as calculated by doering. (Keeping in mind that I did confirm ovulation the cycle before so my period is safe. However, there is no way that Natural Cycles could have known that).

september 1

October Cycle (Second with Natural Cycles)

late september nc cycle

For the second time, Natural Cycles gives me a green light on the second day of a temperature shift in the morning. My egg could still be around! In fact, this green light was less than 24 hours after my peak day!

october 1 cycle

November Cycle (Third with Natural Cycles)

late october nc cycle

During this cycle, Natural Cycles decides that I’m not safe on CD 5 (I am).

It also gives me a green light on CD20. However, I would not be safe to have unprotected sex until the evening of CD21 due to peak day occurring on the first day of my shift.

oct 27 cycle


Natural Cycles does not follow the rules for STM charting.

When charting with STM, one must always wait until the evening of P + 3 and T +3 (three high temperatures and three days after peak). Both rules have to be met before you can have any unprotected sex. With weaker shifts, this wait can be a day longer.

Natural Cycles does not include cervical mucus. Since cervical mucus is what opens the fertile window (and not temperatures), this causes the method to lose efficacy. Cervical mucus is what allows sperm to survive and fertilize an egg. When cervical mucus dries up for 3 days and 3 high temperatures above the coverline occur, ovulation is confirmed for the cycle. Any app that leaves out cervical mucus, but still lets women have green days pre-ovulation, is misleading women and putting them at extra risk of pregnancy.

Another worrisome aspect of the app is that it frequently gave me a green light on the morning of the second high temperature. Temperatures can easily be disrupted, and new charters may not know their normal temperature ranges. Women need to be certain that ovulation is over before having unprotected sex. The third high temp (along with the third day after peak day) lets you know that ovulation is confirmed. Green lights after only one or two temps can put women at a risk of pregnancy, especially when cervical mucus isn’t taken into consideration. In fact, most STM methods require 4 high temps for women who don’t use cervical mucus (or cervical position). The reason that Natural Cycles may interpret shifts wrong is because of its static coverline. Some women may not see much of a change in coverlines from cycle to cycle. However, others may see a change. Having a static coverline can give users green days before they actually have a temperature shift.

Lastly, the app is misleading because it marks the day of ovulation. The only way to truly determine the exact day of ovulation is with a well-timed ultrasound. In fact, ovulation is most likely to occur over a period of about 4 days. Here is the link to the study that discusses which 4 days are the most likely. Since ovulation is likely up to day 2 of a temperature shift, this proves even further why it is so risky for Natural Cycles to give green lights on day 2 of a shift. In the other 9% of cases not hightlighted in the green box below, ovulation happened up to 3 or 4 days before a shift. This is why mucus is so important to record pre-confirmed ovulation. If there is mucus present, the sperm may live. Natural Cycles does not take enough factors into account when drawing its fertile window.


If you are coming off of hormonal birth control or postpartum, this app may be an even worse choice for you. Those coming off hormonal birth control and postpartum may experience cycles changing in length and unexpected early ovulation even more so than people with regular cycles.

Anecdotally, I have seen many risky charts from Natural Cycles in the group I help moderate. You can find a link to this group here.

I highly recommend learning a real sympto-thermal method from an instructor and not wasting your money on the Natural Cycles app. I found it quite shocking that is was not very conservative when first learning about my cycles.

Check out my last post to learn how to start charting on a budget. Click here to sign up to learn to chart with me when my course opens in September 2019.

Here are some other articles on Natural Cycles that I recommend reading:

Natural Cycles’ FDA Approval: What’s The Big Deal?

5 Reasons I Don’t Use Daysy Or Natural Cycles

‘I felt colossally naive’: The Backlash Against the Birth Control App