I'm so sorry to hear of this situation, and your story struck a nerve. A good friend of mine went through a situation very similar to yours (her H left her for an OW later in the pregnancy than where you are). There's a lot of good advice here but here's another thought.
Have you thought to use your husband's phone and texting the OW that "This is H's wife. I am XXX weeks pregnant."
Nothing more, but inform her as it seems you think your H hasn't yet told her. It might run counter to some of the advice given here, but here's the thing.
It worked.
Her H came back and while they still have issues, his general tone after the fact was he was glad his wife had the courage to do that. He realized he was being selfish and couldn't see how it was hurting his family.
