I think what you came up with is good. But take a look at these if you are looking for more ideas:
Welcome baby ________!
Mom's got a bun in the oven...Congratulations!
It's a boy/girl!
Or you can use some of the sayings that can also be used for cakes. You can find them on this page:
http://www.best-baby-shower-planning-guide.com/baby-shower-cake-inscriptions.html