Instead of saying "WE" are pregnant, why don't you try saying "YOU" are pregnant. The confusion will stun him and he won't even remember how to get angry if he wanted to.

You: "YOU are pregnant."
Him: "What do you mean I'm pregnant?"
You: "The doctor said you're having a baby. I want to go on Maurey because I don't think it's mine. I want a DNA MATERNITY test."
Him: "It's totally yours!"
[You start to get upset]
Him: "Please don't be mad. I'm sensitive to that."
You: "Sorry, sorry. Anyway, I hope you know I'm going to do the right thing and be there for this child."
Him: "Thank you, I love you. Let me buy you dinner tonight and give you a massage and make you a cake from scratch."
You: "You should take a lamaze class."

See how it works?