I'm a big fan of reproductive rights. Women should have access to the contraception that suits them - and to free, safe and legal abortion services if they want them - but I have my limits.
Some people have called me irresponsible for using and promoting this method. I have to say I would not do either of those things if I thought it would routinely fail one in four women.
But that 76% statistic is a crude tool.
At best it's a handy reminder to do things right. At worst it's an unfair way to scare women (and clinicians) off fertility awareness/natural contraception.
That said, at first glance, it seems completely fair.
It comes from the same source as the effectiveness rates for other kinds of contraception - this review of the evidence by James Trussell.
What could be wrong with that?
The main thing is the lumping together of all the different fertility awareness methods.
Fertility awareness includes a very broad range of methods.
It goes from finger-crossing, to calendar checking, to those who check their waking temperature and/or fluid.
Most of those in the effectiveness review group were using the calendar method. Only one in five of them were also checking their waking temperature and/or fluid.
So the 76% statistic only applies to the calendar method - using your cycle day to work out whether or not you are fertile.
I don't want to dismiss the calendar method. The simplicity of CycleBeads is glorious, but you can generally get better results if you also take your waking temperature - and ideally check your fluid.
I don't want to dismiss statisticians either. I know they have reasons for lumping together all the different fertility awareness users (mainly the tiny number of women that use fertility awareness).
But it is rather wearing to see this 76% figure trotted out again and again.
So what is a fairer statistic? Is it the 99% perfect use figure - also quoted by Trussell and by NHS Choices.
Yes, I think that's a reasonable expectation.
That 99% is based on a study of 900 women using the symptothermal" (temperature and fluid) method of fertility awareness for at least a year. Read the Frank Herrmann et al research here.
Those women might be unusual - they were prepared to be studied for starters, and they had support - but I think that's a fair statistic.
You don't have to be perfect to get perfect use effectiveness.
If you are prepared to learn a few things, record a few things, and handle your fertile days, then 99% effectiveness is a perfectly reasonable expectation.
Interested? Start here.