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Yes it is possible to get pregnant while on implanon and in some cases ectopic pregnancy ( outside uterus in the fallopian tubes) is possible .
It is possible if implanon has been inserted since longer time and also if certain antibiotics are used concurrently which interact with liver enzyme that metabolizes implanon active ingredient( etanogestril).
But again the chances are there but really low.
In case of any doubt, if there are any symptoms of pregnancy, or lower abdominal discomfort or pain, it must be ruled out preferaably by blood test in earliest possible time.