I do not envy the problem you have. It sounds like it could be potentially dangerous. What country are you in? Depending on where you are located currently, I would not necessarily recommend that you tell your doctor that you have the implant. The Hippocratic oath does not necessarily extend to women's reproductive rights in many countries... as you well know.
At this point, what would be the best outcome? To dissolve the marriage, to stay in the marriage, to be honest and come clean about the implant etc...? When you have the implant put in, did you hope that if you did not get pregnant that you would be released from the marriage? Do you have any personal freedom? Could you possibly reply on your parents for help?
I am very worried about you... it sounds like you are a smart woman in a very difficult situation, trying to walk a fine line between health & safety and tradition/religion.
I often feel pulled between what my family requires of me (or expects of me) and what I actually want. I worry that if I express my thoughts and feelings, that I may be punished for it, or that my family will disown me. It sounds like you may be in a similar situation. I'd like to help research for you, but I will need more details.
If you do go to the dr, tell him that you do have an implant in, then tell him the reason why. If he practices patient/doctor confidentiality, he cannot tell anyone what you discuss. If he doesnt do confidentiality, dont go. I hope you find a solution to your problem, seeing as arranged marriages are wrong in my eyes.
I don't know enough about implants to know if a doctor could tell (for example from your hormone levels) that you must have an implant. He or she might read your levels and then think of it, but if you were to say you didn't have one, I don't know if the doc would just think the high hormone levels are from something else. But is the implant not something that can be seen by looking at the skin?
If you are thinking you would like to be out of the marriage, why don't you start the discussion or negotiation to get out, now? Tell them it's not worth a doctor appointment because you have been thinking your husband needs a different wife anyway. If you know it is not working, there is no time like the present to begin the process of getting out.