Technically if you matched, they cannot refuse to send you a contract because of your pregnancy.
However, my answer really depends on when you are due. If you are just barely pregnant, then waiting a month or two to get your contract probably won't make a big deal of a difference. But if you are delivering in July, you should let them know ASAP. One could imagine them trying to shaft you adn refusing to send you a contract.
I'd call and see when they are sending contracts out...if its soon, then wait. Otherwise, let them know sooner rather than later.