In practical terms there are no Safe days, only days of varying probability. Bodies aren't clockwork, and will readily fall out of synch for just about any reason. 20% of women relying on counting days end up pregnant within one year.
after periods
No, they haven't. And women's safe periods isn't that safe either.
they are safe as long as not worn for extreme long periods.
Three days
Oh yes. Bodies aren't clockwork, so "safe periods" aren't particularly safe, they only improve the odds a bit. One out of five women relying on "safe periods" still get pregnant within one year.
Safe Now - 2010 was released on: USA: 2 March 2010
Yes, safe period aren't safe only periods of varying probability. Out of couples relying on safe periods a full 19% gets pregnant during the first year.
a patched tire can be safe to drive on for short periods of time, but shouldn't be driven at highway speeds as they aren't as safe
It is safe to use sauna every day, as long as you don't do it for long periods at a time.
Safe - 2010 II was released on: USA: 14 July 2010 (48 Hour Film Project)
Safe Haven - 2010 was released on: USA: 30 October 2010 (Philadelphia Short Film Festival)
"Safe" periods aren't safe, just for the reason you have described. Bodies aren't clockwork, so even people with "regular" periods can't rely on counting days. One out of five people using "safe" periods as a contraceptive method end up pregnant within one year. Be responsible, get real contraceptives and learn to use them right.