Legal Question in Family Law in California

If you have not stayed under the same roof or had sex with your ex since before the baby could have been conceived, the baby is not even theoretically your ex's. Still, you have a messy situation that should be cleared up. If the new man is "steping up to the plate" you need to both sign a Declaration of Paternity while you are still in the hospital. The maternity staff will have the paperwork for you to do that if you ask (and will offer even if you don't ask if they are on top of things). That will supersede any presumption that the child of a married couple is the child of the husband. If the ex wanted to make trouble after that, he could still claim the presumption, but in the face of the facts and the Declaration he wouldn't get very far. Now if the new man is not accepting responsibility, then you have a much bigger problem. Re-post your question with that fact in it to get an answer about what to do then.