Unless you have a contract that says otherwise, the employer can treat you the same as all other employees and in most way, it must treat you the same. Of course it must comply with all laws regarding pregnancy leave and pregnancy discrimination. If your employer is changing everyone's job and reducing everyone's pay, it is entitled to do the same to you. I recognize this is a tough situation as you probably incorporated your pay into your family planning. But the employer could have legally fired you, too, unless doing so was in violation of a contract or because of your pregnancy. You can certainly ask the employer to leave you on the same pay scale until your maternity leave is over, but you should know that it does not have to do this and may be worried about "reverse discrimination" if it gives you a benefit it does not give to anyone else.
Also, employers generally dislike employees asking for more than the employer is willing to give. Without knowing the personnel involved, I cannot make any predictions abouot what your employer might do. But you may find yourself out of a job in this terrible economy and unable to find replacement employment.
For an overview of pregnancy protection for employees, please look at my Avvo guide on pregnancy discrimination: http://www.avvo.com/pages/show?category_id=6&permalink=pregnancy-leave-federal-and-california-rights
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There is no such thing as maternity leave in California. The law requires an employer to provide unpaid time of for disability due to pregnancy, up to 4 months (17.33 week).
There are new Pregnancy Disability Regulations in California. I believe they would allow the employer to make the change while you are on a pregnancy disability leave. While an employer cannot discriminate due to pregnancy disability, the employee on leave is not entitled to any greater rights than what she would have if she was still at the job.