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Dec 30, 20233 min read

How Effective is NFP?-- Shining a Light on Flawed Studies



How effective is NFP?

Through the years, NFP has gotten a bad reputation as being ineffective when used to avoid pregnancy.  A large part of why NFP got this inaccurate representation was due to a flawed study that the CDC originally cited regarding NFP effectiveness.  This was a retrospective study that surveyed 18,000 women between 1995-2002.  This survey asked women to recall what method of birth control they were using when they got pregnant. Anything that sounded remotely like NFP was put into one “periodic abstinence” category. This means that the rhythm method, cervical mucus method, symptom-hormonal, and symptom-thermal methods were all lumped together.  The results of the study, which were cited by the CDC, reported NFP to have a 24% failure rate.  This means that 24 out of 100 women would end up with unintended pregnancies if using NFP methods.  However, more recent and strong studies show that NFP methods are actually 95.25% - 99.86% effective at preventing pregnancy (or only a 4.75% - 0.14% failure rate), depending on the method that is used ( [1]; [2]).



There were multiple issues with this original study!

First of all, this study was retrospective and contained recall bias.  This is because women could have misremembered what method of NFP they were using at the time of conception since it was asking them to remember things from years prior.  Additionally, they may have been using a mix of methods and forgot that this may have been the case at the time of conception.


Second of all, some methods of NFP are more effective than others.  Therefore, lumping them all together into one category can skew the effectiveness of each.  For example, the rhythm method is much less effective and reliable than more modern NFP methods such as Creighton, so including them all in the same category would portray the effectiveness of the Creighton method as falsely lower than its actual effectiveness.  This would be similar to lumping all hormonal birth control methods together.  Oral contraceptive pills have a 7% failure rate, whereas a hormonal IUD has a 0.1-0.4% failure rate.  Lumping them together in the same category would make IUDs appear far less effective. 


Finally, since this study surveyed women between 1995-2002, these women had to recall the method they were using years prior to the time in which they were surveyed.  Many modern NFP methods were not as popular in practice yet, and there were less scientific advances in NFP at that time.  In fact, 86% of women in the “periodic abstinence” category reported using the rhythm method (a much older and less developed method, which isn’t even a true NFP method).



Recent progress for NFP in the media

In 2019, the CDC changed its statement online regarding NFP effectiveness.  It cited a newer study that was performed in 2018.   This new study reviewed 53 previous studies that measured pregnancy rates of women using FABMs (fertility awareness based methods) and rated the studies based on a set of quality standards.  Using this new study, the CDC changed the quoted “24% failure rate” of NFP methods to 2-23%.  While there is still room for improvement and progress within the CDC’s published material on NFP, this is still a step in the right direction.  It shows that not all NFP methods are equal, and therefore, effectiveness is shown to be a range.  However, one way the CDC could improve their published NFP information would be to specify effectiveness rates for each NFP category. 

 

 


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