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If you are sexually active without protection, and not on any regular birth control measures, then the symptoms (delayed periods, abdominal cramps, back pain) could suggest pregnancy possibility. Do not depend on appearance of classical symptoms alone to suggest possibility of pregnancy. The appearance of symptoms suggestive of pregnancy depends on how your body reacts to the changing hormonal status/levels during pregnancy. If periods have not started within 10 days of expected period dates, or if you have got abnormal periods, or if you suspect chances of pregnancy, consider visiting your doctor/gynecologist at the earliest for thorough examination (to confirm pregnancy related changes/signs in body) and blood/serum HCG level test (to confirm/rule out pregnancy accurately).
Take care.