Section 49-2-310 (5) , MCA provides that no employer may require an employee to take a mandatory maternity leave for an unreasonable length of time. The reasonableness of the length of time for which an employee is required to take a mandatory maternity leave shall be determined on a case by case basis. However, the employer shall have the burden of proving that a maternity leave for a period of time other than that prescribed by the employee's medical doctor is reasonable, and in no case shall an employee be required to take an uncompensated maternity leave for a longer period of time than a medical doctor who has actually examined the employee shall certify that the employee is unable to perform her employment duties. Neither this section nor any other section of these regulations shall prohibit an employer and employee from mutually agreeing, in the case of the particular employee, to a longer period of maternity leave, either compensated or uncompensated than would be required by this regulation. However, no employer may enter into a general agreement with any group or association of employees which requires a longer period of mandatory maternity leave than is permitted by this regulation.