Do I come clean now or after the wedding? (35f) (35m)

We have been together almost 15 months. My relationship started with a pretty big lie and I'm not sure how to deal with it. I dealt with years of infertility I never thought I would become a mother. I am in my 30s. My sister is also in her 30s and gay. She was sexually assaulted, and became pregnant. When she discovered she was pregnant, she was already in her second trimester and she did not want to terminate the pregnancy. We were abused as children and there's a lot of abuse in our family. My sister did not want to explain what had happened. Our family is spread out and not in the same area. Long story short, we told everyone the child was mine. Fast forward. I met someone and it's the best relationship I've ever been in. I assumed I would never find love. I'm not conventionally attractive, I'm not smart, I'm not wealthy. I've met a genuinely great guy who comes from a really nice family. I'm pretty sure he is going to propose. He thinks I gave birth to my child. I told him that I can't conceive naturally and that I had to use donor sperm. I feel badly for keeping this from him. My own mother thinks that I gave birth to this child. I'm not sure how to handle this. Do I tell him now? Or do I wait until we're married?