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There can be chances of contraceptive failures. Emergency contraceptive pills usually work in preventing pregnancy if taken within 48 hours of unprotected intercourse (provided you were not sexually active without protection after taking it). Since the emergency contraceptives contain high dosages of hormones, these can at times cause period pattern abnormalities and delayed periods. Visit 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). If pregnancy gets confirmed, consider getting serial HCG levels estimated. Doubling up of the hormone levels in 48-72 hours will show that pregnancy is going on smoothly. An early ultrasound scan done will help to know if pregnancy is intrauterine or ectopic along with confirmation of gestational age and viability. Be in regular monitoring and follow-up with your treating doctor/gynecologist and report any new/abnormal symptoms immediately.
Take care.