i was wrong by a factor of 2, not 5 as you suggest

also i doubt that 11 days is typical for a woman with an average 28 day cycle (which is more than the majority), i'd guess it's more like 7-9 days

i've witnessed this stuff in real time in a woman who was taking synthetic fertility drugs, and i remember it was about a week that her mucus went waxy and smelled funny... sex wasn't as fun in this time either

and anyhow, the upshot of this is that sampling from a random day on any given month, it's at best a 1/3 chance you might impregnate a woman, 50/50 if you include that she's not going to do sex while she's bleeding, since the end of the cycle it's unlikely to take (takes around 36 hours) and you can offset that first 24 hours as being the time when it's possible the ovum arrives while the sperm are still viable

the point is that nobody is more qualified to evaluate this and selling women a pill to take every day that they could just instead do a thermometer and mucus test, come on, whose interests are served by this perversion of the mothering instincts?