Except as provided in subsection (b) of this section, an eligible employee is entitled to a total of 6 workweeks of unpaid parental leave during any 12–month period for one or more of the following: the birth of a child of the employee; or the placement of a child with the employee for adoption or foster care. An employer may deny unpaid parental leave to an eligible employee if: the denial is necessary to prevent substantial and grievous economic injury to the operations of the employer; and the employer notifies the employee of the denial before the employee begins taking the leave. If an employer provides paid leave to an eligible employee, the employer may require the eligible employee, or the eligible employee may elect, to substitute the paid leave for any part of or all of the period of parental leave.