I believe it is Federal Law, as stated by many responders.
I can see a situation where an employer is looking to fill a position with somebody who will probably stay for many years. I agree there are no guarantees. However, if you hire a married woman of child-bearing age, you almost guarantee that you will have to do a job-search again. If the position is easy to fill, perhaps that is ok. However, if the position you are trying to fill is key to the business, I would not fault an employer for exercising his/her judgment. Even if that judgment subjects the employer to a potential lawsuit.