Do birth control pills stop your fertility immediately?

If timing is right: Birth control pills work in the first cycle you take them as long as 1. You start at the right time in your cycle and 2. You don't miss pills. In the past women were told to start pills on the first sunday after her period starts (to avoid periods on the weekend). It makes more sense to start on day 4 - count the first day of your period as day 1. If you start after day 5 use condoms the 1st month.

BCP stop fertility: If you start bcp's at the beginning of your menses it is most unlikely that you won't ovulate that cycle. Because there is a 5-10% chance that you may still ovulate the first cycle it is wise to use a backup contraceptive like foam and condoms the first month.