In all honesty, you cannot know until the baby is born and your husband has a DNA test. I know thats been said up thread, but honestly - just repeat that to yourself like a mantra, because its true and you will not get the resolution you're looking for until the baby is born (if she chooses to continue with the pregnancy).
If she is going to terminate anyway, I would try not to focus on her cycle and dates, and address the issue of your husband cheating on you and doing so without protection.
I also think those who are accusing the OW of lying should probably reserve judgement. The proven liar in this equation is the OP's husband.
I'm just thinking of a very good friend who was involved in a similar situation, but s the OW. Affair with a MM (well, he actually spun her the old 'we are separated' line), she ended up pregnant, he made her out to be a lunatic to his wife and gave a load of old guff about dates and how it couldn't possibly be his. He and his wife froze out my friend as a lying, unstable bitch and 'worked on their marriage', accusing my friend of being a liar because she refused to show him scan pictures or give him details of appointments (based on the fact he was a cunt).
Lo and behold, the child was born and it (he) was his. So much damage done.
Keep your dignity, OP, and focus on your husbands actions.