Are you planning on staying married? Are the two of you in counseling together? If not, you need to be, desperately.
I would suggest a paternity test (not sure if those can be done before the baby is born, I think they can) And I would also suggest getting a lawyer. If the baby is his and you plan to stay married, the two of you will be paying child support. And from the sounds of this woman, child support to her will be the least of your concerns.

Tough spot you are in, I hope everything works out. The cynic in me wants to tell you to look out for yourself first!!