Having to smile in the face of mountains of baby gear and ultrasound pictures when you're desperately trying to conceive is more than you should expect of yourself -- and more than anyone should expect from you. Unless the baby shower is for an extremely close relative or best friend, simply don't go.
There are several ways to get out of these invitations -- and those who know what you're going through will (or should!) understand. You can choose to be honest with the hostess and explain that these events are just too tough for you right now, or you can make up a reasonable excuse, like your husband's cousin's wedding or some such. If you're really under a lot of pressure to attend, brainstorm a game-time excuse, like stomach flu (no pregnant woman wants to be around someone who's contagious with that).
No matter how you decline, be sure to send a card apologizing for your absence along with a thoughtful gift, like a mini library of books you loved as a kid (a great idea especially if shopping at the big-box baby stores is something you'd rather avoid as well).