Everybody is obviously different, but in my experience, my drive went through the roof during my three pregnancies!!! Take note however that your wife may be worrying that sex could harm the baby...which obviously it can't, but pregnancy is a funny thing, you're growing this person and you are so protective of it!
Also, if she has any morning sickness that does not make you feel like sex or sexy at all! It could be her changing body that is also putting her off or I know in my best friends experience she said it felt wrong as though the baby was involved.....which is irrational but pregnancy does tend to bring out crazy thoughts!!!
Just give her time, lots of compliments, kiss her, tell her you love her and just go with the flow.....generally when second trimester hits the "glow" hits and you get lots of pregnancy energy and horny feelings explode!!!