sadsadmom,
Calm down and talk to your husband first. Try to find out the reason why he did that, see if he wants to rescue this marriage or not. If yes, work out a plan - how and when he would cut the tie with that woman, what to do to welcome your new baby and so and so. There'll be a lot changes of the parents' lives once the baby arrives. If he still wants to be around with you and the baby, he need to know that there'd be a lot of works that you need him to share soon to take care of the newborn. Both of you have to find out what goes wrong and decide how to do to fix it. You can consider a marriage counselling.
If he does not treasure this marriage, you need to start to plan for yourself and your newborn. Do you have close friends that you can talk to and who can provide help? As you know, there're a lot of things to prepare for the newborn when it gets closer to your due date.
I have a friend who has similar experience but her was even worse. She has been married for 13 years. Their baby bornt in 2005 and she discovered that her husband had cheated on her for 3 years 1 week after her son's 2nd birthday. It meant that he started the affair before she was pregnant and it continued during her pregnancy and after the son's bornt. She's so upset and kicked him out the night that she found out the truth. Her husband really wanted to rescue the marriage but he had missed so much opportunities that she has given before. Finally, they divorced this year and they spent the whole year to prepare - financial issues, childcare, relationship, etc. They are still good friends but no longer be couple. One thing we are doing good is to keep seeing each other every week and the son still receive love from both mom and dad and their families. Honestly speaking, a complete family is the best for a kid's growth but a happy relationship is the key.
Voice out anytime if you need help or just someone to talk to.
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