If the pack contains 21 pills you should not make a pause of 7 days but start a new pack right after you finished the previous pack in order to prevent the expected period. If the pack contains 28 pills you should throw out the last 7 (inactive) pills and start a new pack on the 22nd day. By not making a pause (or by replacing the last 7 inactive pills with active pills from the new pack) estrogen levels remain high enough to prevent the expected period. This manipulation of birth control pills is safe if not done very often because if it is done often, it can cause irregular and severe bleeding from the womb.