Understanding how one’s body and cycle work can be a very empowering experience. While Taking Charge of Your Fertility (TCOYF) is a great book to get further insight and learn about how the body works and how to track fertility parameters, such as how cervical mucus changes throughout the cycle, it is very different from Natural Cycles in many ways.
The TCOYF “rules”
In the book, TCOYF provides “rules” to follow in order to take charge of one’s fertility. They are slightly modified from the so-called Symptothermal Method (STM), which combines cervical mucus tracking with basic, manual temperature tracking.
Limitations of the Symptothermal Method
The Symptothermal Method has a high risk of error for two reasons:
- Establishing whether one is fertile or not at the beginning of the cycle solely based on cervical mucus requires a lot of training, which puts a lot of responsibility on the individual and increases the risk of error.
- The rudimentary rules to determine a temperature shift are highly error-prone and do not take statistics or previous cycles into account. The method relies on single temperature values, which is not reliable as many things can affect a single night’s reading. This is problematic to get wrong as the potential of mistaking highly fertile days for infertile near or during ovulation is high.
The Symptothermal Method’s effectiveness
In general, the effectiveness of the TCOYF rules has never been clinically validated. The book refers to the Contraceptive Technology book, and claims based on this that the Symptothermal Method has a 2% typical use failure rate. This is based on a single German study, and the information is misleading and taken out of context. TCOYF omits the fact that the Contraceptive Technology book also mentions that in addition to the German study, “multiple other studies generated substantially higher estimates, up to 33% for typical use”. The Contraceptive Technology book also states that “We find the results for the Symptothermal Method (study conducted in Germany) unbelievable, although we cannot identify any errors in study design or analysis.”
Reading the German study in more detail reveals that the women in the study had to take a three-month-long in-person course before they entered the study. This means that the participants of the study were highly trained on the method as well as highly dedicated, and are a poor example of real-life situations. This is confirmed by the very low rate of women having unprotected intercourse on days marked as fertile in the study, which is different from what we see among our users in real life. The study also excludes women that, for instance, have irregular cycles. The slightly modified rules in TCOYF have never been studied.
Natural Cycles’ algorithm
Natural Cycles is different from TCOYF as the NC° algorithm learns over time and adapts to each person’s unique cycle. Natural Cycles is effective from the first day of use. Learn how here.
Natural Cycles’ effectiveness
Natural Cycles' algorithm has been cleared as a medical device for birth control both by the FDA in the US and in Europe, which means that rigorous clinical studies have been performed to establish the effectiveness. Natural Cycles' algorithm’s method failure is 0.5%, while the perfect use effectiveness was shown to be 98% (2% failure rate), and the typical use effectiveness is 93% (7% failure rate).
After a lot of scrutiny, the FDA also confirmed that Natural Cycles is as effective for those with irregular cycles as for those with regular ones.
Other differences between Natural Cycles & TCOYF
- The effectiveness of the TCOYF rules has never been clinically validated. However, as a regulated medical device, Natural Cycles is subject to rigorous testing and yearly audits. Our FDA clearance is based on the analysis of more than 15,000 women over 180,000 cycles. We monitor the app’s performance monthly to ensure our Pearl Index remains constant and our effectiveness rate remains the same. You can read more about our research, including our effectiveness studies, here.
- The TCOYF states that if one only uses temperature and not cervical mucus, one should not trust pre-ovulatory non-fertile (green) days as one cannot predict ovulation in advance based on temperature only. However, the Natural Cycles algorithm learns from a user's individual cycle pattern and has a generous buffer of red days before the ovulation day, taking any cycle irregularities into account. When using Natural Cycles, green days can be trusted both before and after ovulation, since the risk of pregnancy is very low due to the algorithm giving a green day in the fertile window (0.5%).
- The Natural Cycles algorithm uses sophisticated statistical tests to confirm that ovulation has happened by comparing the temperature curve to what that particular person’s ovulation pattern and temperature pattern look like during ovulation. Therefore, it can sometimes need more than three high temperatures and sometimes less, depending on the data and statistical significance. If Natural Cycles instead used the simple 3-over-6 rule of TCOYF to confirm ovulation, the method failure rate would be much higher than it is.