This one should be a no-brainer. If the bride and groom are hiding a pregnancy, it's not your job to blow their cover. Imagine the horror on their families' faces if you were the one to drop the bombshell by saying something like, "Did you guys register for that bun you have in the oven?" No doubt, the newlyweds are waiting for the right time to break the news, so don't ruin it for them!
You should actually consider it a privilege that you know such confidential information, and do what it takes to protect the secret. No matter the circumstances, there are some things that just aren't your responsibility to share.