When can I let my baby play with a pet dog?

1 days. I answer this as a father and dog owner/enthusiest. Dogs I have owned have been protective and accepting of my children right away. We have had kid friendly dogs (golden's, labs, westies) one can never be entirely sure whether a dog may "snap" at a baby or child, however, so close observation is warranted. I would not recommend a pit bull or guard breed have contact with any child.
Formal introduction. Depends on the dog and his/her temperament.Do it slowly, be cautious as your dog will be curious and at time jealous.As a rule of thumb.It is never a good idea to leave baby alone with your pet;better safe than sorry.A healthy beginning could lead to a healthy and beautiful relationship.
3-4 days. If your infant and the dog are being supervised, and the dog is up to date on his immunizations, there really is no early age limit. Your infant can be exposed to your dog as soon as your baby comes home from the hospital. If your dog goes outside, this may be an opportunity to enhance your infant's immune system by exposing the baby to allergens in small amounts.