How To Tell The Little One

I'm 10 weeks & 2 days except my stepdaughter doesn't know that. Me and my fiancé really don't know how to tell her in creative way let alone knowing if she'll take it well, simply because she can be very complicated for a 3 year old, surprisingly. My fiancé was telling me how she'll ask questions like: "Why is mommy always sick?", "Why is does mommy's tummy look different?".. I love her to death and I just don't wanna make her feel like she's being replaced or anything like that, and I just want her to feel comfortabe about it. Please help!