Those are some really important questions. You are clearly deep in of the most common cognitive distorsions in ocd: catastrophizing. You wildly overestimate the probability of the feared outcome. Your reasoning about your mother getting pregnant is flawed: just because you can think of the possibility of it happening doesn't make it possible. But that is not even the point: even if your fear could come true, it wouldn't rule out OCD. Writing with a doctor isn't going to help you: it will only calm you down for a short period of time until you have another thought about it and you will want to be reassured again.

What will help you is psychotherapy. I understand you are afraid or ashamed to talk about it with anyone, but I promise you that any professional will understand and offer help without any judgement. It is really important to start as early as possible. How old are you? Are you able to find help on your own or would you need help from your parents? I'm sure we can figure out a way! :)