I'm 25, pregnant, and I dont know how to tell my parents?

Hi and thanks for checking out my question. So I am 25 years old and I am 7 weeks pregnant. I'm living with my fiance and were been together for 5 1/2 years. This is the thing, my parents have told me before that they dont want grandchildren and that I shouldnt have any children. They say this because I have bipolar disorder. I do get moody yes but the episodes are under much better control now. I do not even know how to broach this with them because I know that they would be upset and dissapointed... and my fiance lost his job and is collecting unemployment right now. Plkease someone give me some real advice? How do I handle this? I am happy to have one on the way but this is bringing a big cloud over everything. Ps I'm on meds but the amt low and its better for me to be on some than none. THanks