Mammals a weird. There have been plenty of examples of fully grown predators caring for baby prey animals for short periods of time. Example:
http://www.youtube.com/watch?v=LdhG5bNMei0
That's some deeply hard coded behaviour... (Particularly for female mammals.)
But in the case of domestic animals it can go the other way where the pet will see the new baby as a rival. I've know of several people who gave away their beloved pets when they were expecting a child for that reason... Well, that and the potential for accidents...