i am not sure i completely understand your question since the main reason women take birth control is so that can have sex at the same time. if you are talking about taking just one pill or your first pill on the same day you had sex this will not be an effective contraceptive. as you know sex causes pregnancy if you are ovulating at the time and your partner is capable of fathering a child. a woman on birth control pills really should complete a full cycle (one month of the pills plus a period) before using it as a contraceptive. using condoms for the first month would offer good insurance.
can taking a birth control pill and having sex at the same time cause pregnancy