I'm so sorry. Committing adultery is enough of a horrible, toxic body-slam to a couple but a possible OC is a whole 'nother layer to the shit-sandwich that you were served. I take it that he's not paying child support or has he hidden that too? Has paternity ever been officially determined by a DNA test? And was this really a ONS? Getting pregnant after one bought of sex certainly can happen, but it's not that common. So was there more? Or if it was a ONS, was it another of her APs that actually got her pregnant?
I would so highly suggest that on Tuesday morning you call a lawyer and find out what your legal rights are and what is likely to happen if this OC is your WHs and if the OW petitions for CS (child support). This is possibly going to impact YOUR children and you need to know what the future looks like.
Some things that you need to think about. If this OC is your WHs, what contact, if any, are you going to be OK with. You can, and should, demand NC with the OW. If he wants to see the OC, then arrangements can be made for a hand-off that does not include the OW being present. If she planned this pregnancy, then she is a definite threat to your marriage.
And if you haven't already, take a look in the upper left corner, in the yellow box, and click on The Healing Library. Start reading, especially the part about the 180. Read any post on this forum that has a bulls-eye next to it. You may need to go to pg 2 to find all of them, but they all have good, sound practicle advice from people who have walking your path. Please come back often for support. We're all here for you. (((hugs)))