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After prolonged use of hormonal contraceptive pills, you can have shorter and lighter periods (along with mild delay in periods) due to hormonal pattern changes and your body getting adjusted to the changes. As long as you start with your periods/withdrawal bleeding by the time you finish with placebo pills, it is considered to be normal. Start with the next birth control pill pack as scheduled after finishing with placebo pills even if the periods/bleeding continues. Visit your doctor/gynecologist for further evaluation and clarification.
Take care.