Read through some threads, there is a lot of this going on? Not just wives, but husbands too.
Is she on the pill? She may just be exhausted or maybe have the 'mommy' syndrome... You know where mommy's are not supposed to be sexy or that we feel guilty if we don't give 100% of our attention to our kids.
The weight may very well be a factor as well. It's hard for us to deal with the changes in our body as our weight increases. But even the slightest thing can help her, going on walks, etc. It's amazing what just a little exercise can do for the way we look at ourselves.