This is one of those questions that is far more complicated than it looks on the surface, so bear with me while I do a bit of explaining. (If science makes you break out in hives, you can skip straight to the last paragraph.) If you look around, you will see a wide range of effectiveness statistics for Fertility Awareness (FA). For example, many websites claim an effectiveness rate of 99.6%, based on a robust study published in 2007. But the CDC gives a failure rate of 2-23%! So what do we make of these differences?
To begin with, understand that for each type of contraception, there are two rates of effectiveness. “Perfect use” is when the method is used perfectly and consistently. Sometimes you will see this expressed as its opposite, i.e. “method failure,” meaning how often the method itself fails (not the person using it). “Typical use” takes into account the fact that people often do not use methods consistently or correctly. When this is expressed as its opposite, it’s called “user failure.”
Obviously, perfect use is always going to better than typical use. For example, let’s look at condoms:
Perfect use: 98% (or 2% method failure)
Typical use: 82% (or 18% user failure)
There is a big difference because a lot of people don’t use condoms consistently or correctly. So when you look at statistics associated with Fertility Awareness Based Methods (FABM), you need to consider whether they are citing perfect use or typical use. The 2-23% figure cited by the CDC refers to typical use.
Another question to ask is “What type of Fertility Awareness are they talking about?” Many people, even some health writers and researchers, mix together statistical methods that have a relatively low effectiveness rate (eg Rhythm or Calendar) with observational methods of Fertility Awareness (see FAQ “Is this the Rhythm Method?”). Sometimes this is due to lack of understanding, but sometimes it’s due to lack of sufficient data. The 99.6% figure is based on a particular, highly effective type of symptothermal Fertility Awareness.
Finally, understand that effectiveness, while important, is not the only factor one should consider when choosing a method of birth control. Abstinence is the most effective method, followed by sterilization, but those are not necessarily the best choices for everyone. There are other factors to consider. Your method of contraception has to be a good fit for you, at this particular point in your life.
The symptothermal method of Fertility Awareness can be 99.6% effective at preventing pregnancy IF certain conditions are met:
-The method is used perfectly. That means keeping good data and always following all the rules of that method.
-High quality instruction is received. The participants in the 2007 study received standardized, personal instruction from trained instructors. They did not learn it from the internet. They did not pick it up through use of an app.
Ultimately, the effectiveness of Fertility Awareness is up to the user. People who choose an effective method, and who learn that method thoroughly and use it conscientiously, are highly unlikely to get pregnant unless they choose to.