It's not suo much about innocent or not innocent with the OW. My friend, for example, has separated from her H. He only moved out recently, almost a year after separating, and was already in another relationship whilst still living in the same house as my friend. She, too, was in a relationship. They were the genuine example of two people who had moved on from their marriage emotionally, but were just waiting for the practical circumstances to play catch up.
Your H, beinh the lying fucking arsehole that he is, could well have fed all of the above to the OW. She may genuinely have believed that he was being monogamous to her alone. In that light, whether or not the pregnancy was planned or not doesn't really matter.
Again, even if she is the devil himself incarnate in blonde Italian form, she is NOT THE ENEMY. She wasn't married to you, she wasn't trying to have a baby with you, she didn't commit to you forever, they were not her vows to break.
Assuming she knew that it was an affair rather than a relationship with a separated man, then obviously she's not blameless.
But HE is the twunt in this situation. He's the one who took all the chutney. Don't forget that, and channel that anger proactively and healthily into taking action to move your life forward in a positive way.