...to prevent pregnancy? i just started birth control to regulate my period today, how long will it take to prevent pregnancy? i had used this method once before to get my period back, but after running out of birth control i couldn't find the time to get a new prescription. so i went almost a year without it. last time my doctor said to just start taking the pills the day i got them because i wasnt getting a period at all. so my prescription was filled today and i took the first pill...how long will the birth control take to prevent pregnancy?