How long has your wife been off?
Did you make any arrangements for STD payments?
I am trying to come up with something to help you but so far from what you have said the company may be fully within their rights to have laid her off.
Also, personally, I think that nothing could be more counterproductive than a lawsuit. They are negative and drag on for years. You should be enjoying your baby, instead. Also, suing an employer often makes it very difficult to get another job in the area. Although this is a surprise, it is not the end of the world.