Maryland Code, State Personnel and Pensions § 7-403

(1) the appointing authority decides that additional time is necessary to allow the employee to demonstrate proficiency in the performance of the employee's assigned duties and responsibilities;

(2) in the appointing authority's judgment, the period of the employee's absence on approved leave is sufficient to prevent the appointing authority from making a reasonable decision regarding the proficiency of the employee's performance; or

(3) the employee requests to extend the probationary period.

(b)(1) Except as provided in paragraph (2) of this subsection, an employee's probationary period may be extended under subsection (a) of this section up to one additional 6-month period.

(2) For an employee appointed to a skilled service position below grade 7, an employee's probationary period may be extended under subsection (a) of this section up to one additional 3-month period.