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False positive results with urine pregnancy test kits are rare (and are not affected by use of hormonal contraceptives). Visit your doctor/gynecologist for examination to look for position of the IUD. If you suspect chances of pregnancy, or if you get abnormal periods, consider visiting your doctor/gynecologist 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 is confirmed, the IUD should be removed immediately. Be in regular monitoring and follow-up with your treating gynecologist and report any unusual symptoms immediately.
Take care.