The pumpkin is the perfect item for your baby shower games. There are a couple of ways that I can think of, off the bat that you can use pumpkins. Here are just a few:
Pick the Pumpkin
In this game you take pumpkins and gourds of various sizes and display them on a table. They can even be used as the center piece and therefore doubling as a decoration. Here you will label the pumpkins with a letter (A, B, C, etc.) You have the guests guess which pumpkin matches which stage of gestational development which is also identified by a small piece of paper. (Examples include: weighs as much as a full term baby, size of the uterus at 12 weeks, where mom-to-be's baby is right now, etc.)
Another interesting way to use pumpkins would be to buy enough mini pumpkins to give one to each guest. Have each of the guests use paint or markers to design a pumpkin baby. Have the mom-to-be select a winner of her choice. Or, if you'd like, you can have a small dot on the bottom of a pumpkin to indicate the winner by chance.
Another way to use the mini pumpkins is to again, have them for everyone, but on the bottom of each pumpkin have someone write a message to mom or baby. Then mom can read them out loud during the shower.