My original answer was incorrect. There *IS* a federal law to protect you called the Pregnancy Discrimination Act which attached itself to Title VII of the Civil Rights Act of 1964.
http://www.eeoc.gov/facts/fs-preg.html
Here's the important bit: "Pregnant employees must be permitted to work as long as they are able to perform their jobs. If an employee has been absent from work as a result of a pregnancy-related condition and recovers, her employer may not require her to remain on leave until the baby's birth. An employer also may not have a rule that prohibits an employee from returning to work for a predetermined length of time after childbirth."
It would help if you mentioned your job. Then we could judge if there was a valid reason for less shifts. If you're a pole dancer or work at a job that requires lifting, they're free to not schedule you.
A job is required to not fire you for being pregnant, however, they are not forced to allow you to switch jobs. They don't have to for instance demand that someone else do the physical parts of your job, while you get the easy parts of theirs.