Not dogs... but when I was 6 months pregnant we lost our cat to feline leukemia that didn't even show up in the autopsy until they removed her liver. It was absolutely heartbreaking. I went to a no-kill shelter and started looking at new kitties to keep Mr. Kitty company. The lady there told me it was a bad idea, and implored me to come back once the baby was born and things had settled down. I didn't listen.
The cat was high high high energy. He was destructive and he was a biter. When Arden was 5 days old, he kept everyone up all night destroying the house, tearing up paper towels, and toilet paper, pulling speakers off shelves--and tearing up my husband's hand (blood everywhere!). I was in tears, trying to deal with all of this--with a newborn. I tried to convince myself that I could play with him more, find a way to discourage the biting. In the end, though, we took him back. It wasn't fair to anyone. Luckily it was a no-kill shelter that keeps cats for life if they're not adopted.
Wait until the baby comes, find some balance in your new, larger family--then get another pet.