If "mom" knows the name of her baby-to-be, guests can write poems using the letters of it's name. For example:

EVE
E is for Everything lovely and sweet.
V is for Virtually all that's neat.
E is for Emeralds precious and rare.
That's why we'll give you so much care.

You want to make sure that these are done with "archival safe" or "acid and lignin free" papers and pens as Mom-to-Be will want to save these.