its a jealousy issue, protective (by nature) & frankly, just kinda pissed off LOL he might be pissed off for not recieving nearly as much attention as before. its probably just a mix of emotions.
when your girl has the baby & brings the lil miracle home for the first time, have the cat around it to get familiar with him/her. it will smell him/her for a while, then most likely, wont be any problem.
kittens & younger cats are hard to have around newborns. its the older "i dont give a fuck about a baby" cats that are great around babys LOL
