When you're using birth control pills, you are always supposed to start the first day of your period, please follow the instructions on the packaging. Otherwise it will be ineffective.
In Wikipedia Edit
Combined oral contraceptive pills provide effective contraception from the very first pill if started within five days of the beginning of the menstrual cycle (within five days of the first day of menstruation). If started at any other time in the menstrual cycle, the pill provides effective contraception only after 7 consecutive days use of active pills, so a backup method of contraception must be used until active pills have been taken for 7 consecutive days.
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