What are my rights if I was not given back the responsibilities that I had prior to taking maternity leave?

First, if you did not use Family and Medical Leave Act leave, leave under an equivalent state law, or leave guaranteed to you by a contract, there is no right to maternity leave; that is, in the absence of the above, an employer does not have to provide maternity leave at all. Since they don't have to provide maternity leave, they may take action--such as changing responsibilities--as a consequnce of leave, if they let you take it.

(Note: to qualify for FMLA leave--and most state laws have similar requirements--the employer must be large enough, which is at least 50 employees located within a 75-mile radius, and you must have worked there, more-or-less full time, for a year.)

If you did take leave pursuant  to FMLA, a similar state law, or a contract, then you should not be punished for using the leave. However, in this context, that means you should not have your pay reduced, benefits taken away, or demoted. It does not mean that your employer may not manage its business, reassign responsibilities, or support other employees, so long as it does not reduce your pay, benefits, or take other such concrete negative action against you. Even when your use of leave is protected, that does not mean that nothing may change.

However, if you did use FMLA or other protected leave and suffered a loss of compensation or benefits, a demotion, etc., you may have a legal claim, and should speak with an employment attorney.