Sperm can live, in the BEST conditions (a lot of good quality EWCM) for about 5 days, 72 hours is more normal. An egg lives about 24hours after ovulation. Conception occuring when either sperm or egg is reaching the end of its life often results in early m/c/chemical PG as the genetic material doesn't survive age well.

In general it would be only those with short cycles and long AF's, or those who have unusually long luteal phases (and ovulate early) who could concieve by having sex while they were having their period. The 2% risk thing is true - meaning sex on AF is a better contraceptive than condoms and almost as good as the Pill (1% risk).

I had a 28/9 day cycle for the 3 years we used natural contraceptive methods, i ovulated usually on day 13-15 and i bled for 5-7 days. I was ALWAYS safe when i was on AF. The month i fell pregnant (we weren't trying) i ovulated late due to a sinus infection.