@cristian1980 From your more detailed description, it sounds like the dog is resource guarding your wife. I have had 2 dogs that developed that habit with me. The difference was that my kid was not a baby, and I had enough experience to deal with it. In your family's situation, I would first look for pit resources in your area. I live outside of a mid sized city, and there are a ton of resources to assist pit owners. The big animal rescue has discounted pit specific training, as well as discounted to free basic health care (shots/spay/neutering) There are also several pit specific rescues and support networks. That doesn't mean they aren't all overwhelmed, but it is worth looking into local resources. If you can't get help, then rehoming the dog is in your child's best interest. It only takes one minute of distraction for tragedy to take place. A new baby, especially once they start moving is hard enough to keep safe. Until the decision has been made, you need to keep the 2 of them separate. Do not assume that if you are in the same room, it is safe. Crate, muzzle train, and give the pup as much one on one love and attention as you can spare. Set him up to be as adoptable as possible, while he is still in your care. But never give him physical access to your child. Best of luck.
*please note that the breed of your dog is only relevant because in many areas, there are breed specific resources. Any breed of dog is a threat to a child if not properly trained, socialized, and supervised.