Mdinana beat me to it, but I was going to suggest the possibility of SAD (Seasonal Affective Disorder), too. On the "bright" side, there is a therapy that seems to help...and get this, it involves...light! I don't know what specific lights/characteristics are needed, but a search for SAD "cures" on the internet should provide a wealth of information.
I also concur with others who've recommended exercise. A must, I think. Also, check what kinds of foods you're eating. IIRC, on the news recently, different study results showed that there are particular foods that supposedly affect "happiness" and "depression."
Caveat: there are studies done on just about everything, and who knows what is true, false or just hype. I don't remember the specific foods mentioned, but as I recall, the bottom line was to eat healthy, fresh foods, and stay away from processed foods...as if we all didn't already know that.
Last but probably most important as others have recommended, discuss this with her doctor. He/she knows your wife's situation better than any of us, and should be able to make some suggestions for fighting off the blues.
Oh, one other thing, is your wife worried and/or preoccupied with something? Like work/job, health, finances, family members, etc.
Probably out of the question, especially with a baby on the way, but I'll throw it out anyway. How about a pet? Doesn't necessarily have to be a dog or cat, maybe even a couple of goldfish?