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Starting the pill in midcycle could affect your hormonal levels and cause bleeding problems either in form of prolonged bleeding or irregular spotting, nausea, headaches, etc. It will also not give you desired protection for pregnancy. Birth control pills work the best when started as instructed since they contain hormones in them which are to regulate the menstrual cycles and prevent ovulation, bring changes in cervical mucus, etc., to prevent pregnancy. Use additional protection (condom/spermicide) if having intercourse until you get your periods to remain protected against pregnancy. Start with the next after your periods as instructed.
Take care.