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Missed periods is most commonly caused by pregnancy if you are sexually active and not on any form of birth control measure. Other possibilities include hormonal imbalances, thyroid hormone issues, ovarian cysts or problems etc. Visit your doctor/gynecologist for complete evaluation that includes hormonal work-up, blood work-up and ultrasound scan. If you have already given samples for test, wait for your test results to come. If you suspect to be pregnant, consider getting a thorough examination (to confirm pregnancy related changes/signs in body) and blood/serum HCG level test (to confirm/rule out pregnancy accurately). Look out for development of any other new symptoms suggestive of early pregnancy.
Take care.