[mountainair]

Sorry, my question is: How does one use symptoms-based perfectly? My understanding is that a perfect use case happens only a few days each month. So on those few days, you might have very low "failure" rates, but on the other 25 days per month, is a "perfect use" possible?

[splat]

Sorry I misinterpreted your question! Just a little background -- the reason symptoms-based works is because it's only possible to conceive for ~12-24 hours per cycle. The uterus can sustain sperm for a period of ~2-5 days, so if there's no sperm in the uterus for this ~6 day period, pregnancy is not possible. There are several indicators that signal the beginning of this period: basal body temperature, cervical mucus consistency, and cervical position. To get the best readings, however, these measurements need to be taken immediately after waking up. Moreover, you need a long period of readings (> 6 months) in order to be able to correctly interpret the measurements. I'm assuming that perfect use in this context consists of taking these measurements consistently over a period of at least six months.

[eli]

By precisely observing symptoms and acting on them appropriately? Wikipedia has a link to the paper the data comes from, if you want their definition.

[sp332]

It's not a few days, it's a week or two. And the "perfect use" on the other days is not to do anything that might get you pregnant!

[gjm11]

In other words, "natural family planning" achieves its very low "perfect use" failure rate by solving a different (and easier) problem from the ones addressed by other methods.

It's easy (in principle) to solve the problem of avoiding pregnancy: "just" avoid all sexual activity. The problem contraceptives were invented to address wasn't the problem of avoiding pregnancy but the problem of having sex without getting pregnant.

"Natural family planning" is -- when done optimally -- very effective at the "without getting pregnant" part of that, but not so good on the "having sex" part, since somewhere around half the time it's equivalent to abstinence.

Redefining the problem isn't always a bad thing, but it's as well to be aware of when it's being done.