Your questions answered.
How Effective Is Fertility Awareness?
How is fertility awareness method measured for effectiveness?
When you search 'fertility awareness method effectiveness' this is the first link you’ll find, from Planned Parenthood.
76%-88% doesn't sound like a reliable method, and many will see this statistic and and dismiss fertility awareness as a viable contraceptive. Sometimes this stat is simply explained using the phrase: '1/4 of couples will get pregnant in their first year using FAMs' and that, reasonably, makes people skeptical.
The 24% failure rate number comes from this CDC table.
FAM based methods, when used typically are listed with a 24% failure rate. Notice that there is complete data for all method types with the exception of the FAM section, in a few key areas.
Let’s start by clarifying the difference between typical and perfect use:
Typical use: how well it works when real people in real life use it.
Perfect use: how well the method works when it’s used in a clinical trial. A properly informed user can greatly improve their rate of effectiveness in this method.
The typical use rates for individual FAMs are missing from this table. One reason for this could be because it is hard to measure typical use of these various methods which have a low number of users & are dependent on informed users.
However, a combined typical use statistic for combined FAMs of 24% is provided.
Also note that the CDC does provide a further breakdown of FAM methods, but only lists the perfect use rate of effectiveness. These perfect use statistics are provided from much smaller and less diverse population sizes than the rest of the table.
Understanding the table and its limitations
The majority of the contraceptive data in this table is provided from the National Survey of Family Growth. It is a larger analysis of population data across a wide spectrum of demographics. Clinical trials which are far smaller sized studies, give us different statistics, but this table is intended to reflect self reported users.
Yet 86% of those who reported using 'fertility awareness based methods' in this typical use data actually report using the rhythm/calendar method specifically. [1]
This makes the 24% typical failure rate highly skewed.
The 24% number provided is more representative of people using the rhythm method than of users practicing reliable fertility awareness based methods such as the sympothermal method. Unfortunately, much less reliable predictive methods, like the rhythm method, are calculated alongside and grouped in with the sympto-thermal method, which relies on diagnostic & retroactive fertility signs.
FAM use is small, representing only about 2-3% of contraceptive users compared to the rest of the contraceptive options. This makes the data more unstable & so instead of eliminating it from the table, the CDC chose to combine to produce a typical use of 24%.
The statistical choice to combine FAMs with calendar methods has resulted in the incorrect assumption that 1 in 4 couples will have an unintended pregnancy using sympto-thermal fertility awareness without ever learning what the method is or how it works.
Although comprised of a smaller population, this is a peer reviewed journal from 2013 which lists typical use rates for a few different kinds of fertility awareness based methods.
The symptothermal method (STM):
0.4 %- 0.6% perfect use failure
1.6%-2.2% typical use failure
Fertility Awareness Study Effectiveness Quality Review
In 2018 the first quality review was conducted on studies on fertility awareness based methods.
"Our comprehensive systematic review of the effectiveness of fertility awareness–based methods for avoiding pregnancy reveals that the current evidence base for each method is small and of low to moderate quality."
"We identified several important limitations of the existing literature. No study achieved a high-quality rating across all 13 criteria (Table 2)."
"Many people believe they are using a fertility awareness–based method to avoid pregnancy when in fact the method they are using has not undergone a standard prospective effectiveness assessment. Several methods shown in italics in Table 1 (including many increasingly popular internet applications) have no prospective trial data available to support their effectiveness yet are nonetheless being marketed as effective ways to avoid pregnancy. We strongly encourage investigation of the effectiveness of any new adaptation of existing fertility awareness–based methods, including internet-based versions. Likewise, users who make their own modifications should be counseled that effectiveness estimates from a specific fertility awareness–based method may not apply to their use of the method."
This review should be considered when choosing which fertility awareness method works for you. The review concludes that we need much more study into fertility awareness based methods, and that none of the studies performed thus far are of high quality.
So we have very different statistics here.
What does it all mean?
The above review ranked the symptothermal method study [Frank-Herrmann] of moderate quality. This is the study where we derive the 99.6% perfect use rate number from, and you may hear people refer to this effectiveness rate when talking about fertility awareness.
The commonly touted 24% failure rate is inaccurate to describe sympto-thermal fertility awareness methods which rely on daily biomarkers. The effectiveness of fertility awareness in practice is often a lot more complicated than it's presented to be.
The CDC has finally, in 2019, changed the range of typical use failure rates on their website. It is now listed as 2-23%, varying between methods, instead of the 24% statistic. Still, this doesn't teach people about body literacy nor does it make any distinction between the vast differences between, say, the rhythm method & the symptothermal method. But it is an improvement to acknowledging that this method is a viable option for menstruating people.
Updated CDC table in 2019 showing the range of typical use failure rates instead of the combined rate.