i have taken the m.a.p lots of time (i'm not bragging about this by the way)
It can make your period late or early, or different than normal, and can make your breasts tender.
It would be very unlikely for you to get pregnant if you had sex in the few days leading up to your period, as your fertile time is about 14 days before the period starts, but one can never tell!
it has always been reliable for me, but that is a misnomer really, as i have never tried to get pregnant before, i dont even know i can. I might just not have been pregnant all those times because im sterile or something. If you are worrying, you can take a test anytime from the first missed day of your period ie now