I have been on birth control for the last 6 months. When I was first prescribed the doctor told me they'd be effective after 4 days. I had unprotected sex after 4 days and was fine. I stopped taking them 3 weeks ago and am just now starting again. My period isn't regular but I usually started between the 14th and 23rd when I was on birth control. It's the 21st and still no period. Could this pill prevent my period from coming and I know the doctor said I can have sex four days after the pill but is that really true?