LucyD, I think in the first steps it is okay to tell a white lie. Get to know the person first and let them get to know you and then tell them about your problem.

I had a similar dilemma when I started dating my now-husband. I carry a mutation that increases the risk of having very sick children by 50%. I told him in the first months of dating because I had strong feelings for him and, at the same time, I didn't want to lie or waste his time in case that would have been a problem for him. I was very worried and I was thinking about it every day before telling him. To my surprise it didn't bother him at all. We are married now and I am not alone with my problem anymore.