The answer is found at 29 C.F.R. § 825.213, one of the regulations that implements the FMLA. Summarized, the regulation states:

(a) An employer may recover its share of health plan premiums during a period of unpaid FMLA leave if the employee fails to return to work after the employee's FMLA leave entitlement has been exhausted or expires, unless the reason the employee does not return is due to either:

(1) The continuation, recurrence, or onset of either a serious health condition of the employee or the employee's family member, or a serious injury or illness of a covered servicemember, which would otherwise entitle the employee to leave under FMLA; or

(2) Other circumstances beyond the employee's control.

Examples of "other circumstances" beyond the employee's control are necessarily broad.  They include, but are not limited to, situations as where a parent chooses to stay home with a newborn child who has a serious health condition; an employee's spouse is unexpectedly transferred to a job location more than 75 miles from the employee's worksite; a relative or individual other than a covered family member has a serious health condition and the employee is needed to provide care.

Other circumstances beyond the employee's control would not include a situation where an employee chooses not to return to work to stay home with a well, newborn child.

(3) When an employee fails to return to work for a reason covered by (a)(1) above, the employer may require medical certification of the condition.

(b) Under some circumstances an employer may elect to maintain other benefits, e.g., life insurance, disability insurance, etc., by paying the employee's (share of) premiums during periods of unpaid FMLA leave. If the employer elects to maintain such benefits during the leave, at the conclusion of leave, the employer is entitled to recover only the costs incurred for paying the employee's share of any premiums whether or not the employee returns to work.

An employee who transfers directly from taking FMLA leave to retirement, or who retires during the first 30 days after the employee returns to work, is deemed to have returned to work.

(d) When an employee elects or an employer requires paid leave to be substituted for FMLA leave, the employer may not recover its share of health insurance or other non-health benefit premiums for any period of FMLA leave covered by paid leave.

(e) The amount that self-insured employers may recover is limited to only the employer's share of allowable premiums as would be calculated under COBRA, excluding the two percent fee for administrative costs.