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).
October Cycle (Second with Natural Cycles)
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!
November Cycle (Third with Natural Cycles)
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.
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.
Here are some other articles on Natural Cycles that I recommend reading: