Dogs

It's a shame that we don't have a thread devoted to man's best friend, so I'm starting one with a question. 

We have a 3 year old German Shepherd named Hammy. We had him trained early on. He has a fantastic temperament and isn't prone to aggression. He's been especially great around our 2 year old nephew and our friend's infant son - he just gets it. 

My wife is 11 weeks pregnant, and his temperament around her has changed a little bit. If someone he isn't familiar with approaches her, he goes on the defensive, lunges, and has even nipped someone with his teeth (never anything like clenching his jaw or breaking skin, but still a behavior we need to correct). So my conclusion is that he knows she's pregnant and has gotten ultra protective. I'm pretty much the only human that he won't care if I approach her, etc. 

Does anyone have any experience with this - our old trainer is up to $3,000 and honestly I don't want to shell out anywhere near that kind of cash if there are some methods I can use at home first. 

Any and all help would be appreciated!