Hi Mom- I need to reply to your post because I was in the almost the exact same situation 14 years ago. I was 9 months pregnant and had a 20 month old when I found out my H had an OW. It has been a long time since then and my marriage didn't make it, but maybe things possible could have turned out differently if I had some of the insights, information and understanding that is shared in this forum. Learn, listen and be as patient as you possibly can.
My H was 30 at the time and I believe that my H's problem was not wanting to grow up. I tried to do whatever I could to keep my marriage together and I have never regreted it. I know when I look at my children and see what they have to go through having to live their lives in two different houses, with parents that have no respect for each other, at least I know I did everything that I could to keep our family together.
Be strong for your children and yourself. Find family and friends to help you through this time that should be one of the happiest of your life but instead is going to be one of the biggest challeges. Try as much as you can to enjoy your new baby and keep in mind all of the joy he will bring you. Keep your focus (as much as you can) on the positives in your life and remember that you will get through this no matter what.