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Having breakthrough bleeding while being on prolonged use of hormonal contraceptive pills is considered to be a common side effect. If you have not been regular in taking your birth control pills at scheduled time daily, you can be at risk for pregnancy (as the effective protection may not be present). Wait and watch for your periods. Visit your doctor/gynecologist for further evaluation and assistance. If you suspect chances of pregnancy, or if pregnancy gets confirmed, stop the birth control use.
Take care.