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Your periods can be delayed due to lactation amenorrhea (it suppresses ovulation and causes missed periods) if you are still exclusively breastfeeding your baby. Other possibilities include possibility of pregnancy (if there was late ovulation in your menstrual cycle). Look out for development of any new symptoms. Wait and watch for your next periods. If your periods do not start within 10 days of expected period dates, visit your doctor/gynecologist for examination (to confirm pregnancy related changes/signs in body) and blood/serum HCG level test (to confirm/rule out pregnancy accurately).
Take care.