Thinking of you OP, I'm 19 weeks pregnant with my 2nd and I've found your story so heartbreaking. You've been extremely strong and brave.
The way he's been completely silent makes me think he knew this was coming - either she's been threatening to for awhile, or she's told him she's done it this weekend before he could talk to you.
Either way, he's both a cheat and a coward and everyone will know it.
Good luck to you with everything and I hope the birth goes well xx