Why does my wife do this ?

My wife is pregnant she's been really depressed lately she cries at everything I never seen her so sad. She's usually not one to cry she's usually always happy. She even cried at the addiction network commercial. Even watching her favorite shows on Destination America channel which talks about haunted places in America and ghost and stuff she loves watching it but it makes her cry now especially when they talk about children. Ive been with her 5 years I don't think I ever seen her cry once till now. Why does she cry so much ? How can I help her feel better ?