Every time I've been pregnant I've noticed that I laugh a lot more heartily. I feel a lot more "jolly" all around.
While pregnant with my first, my brother told me he'd decided that to ensure I would have a happy baby, I need to laugh for one whole minute straight. With his antics I easily complied (I think he laughed just so I would laugh).
I think about that when I get a great laugh in while I'm pregnant--just working on making a happy baby. It's got to be good for the stomach muscles too, right?