As Stone Cold and Carlos both said, leave the country, get married there, fly back in with your marriage certificate and it's not a problem.
As long as she is married when she starts to show and (of course) when she gives birth, the authorities here aren't going to count backwards to determine the moment of conception and whether you were married then.
Good luck!

"If you're looking for sympathy, you'll find it between sh*t and syphilis in the dictionary."
- David Sedaris