Why is back-up contraception needed even if I'm on the pill?

Dear Ask The Doctor: I've been on the pill for four months now, I was just wondering after you finish taking the white sugar pills (finish your period) and go onto the active pills, how many do you have to take before you can have unprotected sex? Like when do they start working properly?

Dear Renae: In general, it is always advisable to have back-up contraception within 5-7 days from starting the active pills. This is a safe interval to assure that the optimal effect of the pill has been reached. Back-up contraception is also advised when there are only 5-7 active pills left. Sperm cells can live up to 3-5 days within the vagina; for example, if the last active pill is taken and sexual intercourse is done on that last day, an egg will be released from the ovary on succeeding days and conception can still happen. It is also noteworthy that pills should be taken at the same time every day. If this will be done, effectivity is at ~99.5%; if not, it is only ~90-95% effective. I do hope I have answered your question and take care always.