Help me be diplomatic with MIL

Rhianna1981

So, we live in California, but my in-laws live in New York. They are planning to come out around the time of the baby's birth- and we are excited to see them. However, we live in a really tiny condo, so they will be staying elsewhere.

Anyway, I just got an e-mail from my mother-in-law. She is planning on staying a week (or more!) longer than the FIL and SIL, and she would like to stay on our sofa. If our house was bigger, we would not have an issue with her staying at our house, but I really think that the idea of having a new baby and a mother-in-law staying on the sofa sounds anything but "helpful." 

How should I approach this diplomatically? I really don't want to hurt her feelings, but I certainly want to be looking forward to their visit instead of dreading it- which I will, if I know that she'll on our sofa for a week.