[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.