Birth control like oral contraceptives, you mean? "Birth control" actually applies to a wide variety of things--condoms, fertility awareness method, etc.
If you're talking about oral contraceptives, they protect you from pregnancy all the time (yes, even when you're on the placebos), so there is no best time. Please note that we advocate using barrier methods (condoms, latex gloves, etc) all the time, even if you're on the pill.
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