I rejoined SW after the birth of my 3rd baby, she was 4 weeks old and i felt great. I had been going to SW when i found out I was pregnant but stopped going purely and simply because I wanted to enjoy my last pregnancy and not worry about my weight in that way.
I have taken my daughter to class every week except one and that week everyone asked where she was.So don't worry about taking the baby with you.
Enjoy your pregnancy and sort the weight gain out afterwards. Congratulations on only putting 7lb on so far