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If you did not have your withdrawal bleeding after you missed taking the scheduled Depo shot, and if you have been sexually active (without any other form of protection) then chances of pregnancy cannot be ruled out completely. Your normal fertility can return back early at times (depending on type and duration of the birth control use) and if you have had an ovulation after your due date for the shot, then you can be pregnant. Do not take the Depo shot if you suspect to be pregnant. Look out for development of any new symptoms if you suspect pregnancy. Consider visiting your doctor/gynecologist at the earliest for further examination (to confirm pregnancy related changes/signs in body) and blood/serum HCG level test (to confirm/rule out pregnancy accurately) before taking the Depo shot. Take adequate rest. Drink plenty of water. Maintain healthy diet.
Take care.