The Family and Medical Leave Act, otherwise known as the FMLA, exists to allow employees up to twelve weeks a year of (unpaid) time off to provide temporary care for loved ones who are unable to care for themselves. Upon returning to work, the employee is required to be reinstituted into his or her old position with the same benefits and salary he or she enjoyed prior to taking leave.
In an ideal setting, the employee would give his or her employer 30 days notice before taking FMLA leave. This gives an employer time to review and approve the request and to arrange for coverage during the employee's absence.
A 30-day notice is all well and good in theory, however, life has an incredibly unhelpful tendency of getting in the way. Illness, accidents and even death do not care about deadlines or timelines or even guidelines. The good news is that you may still be eligible for FMLA leave in the event of an emergency.
In an emergency situation such as the onset of a severe illness, an accident or premature labor, you are not going to be able to provide your employer with 30 days notice. Instead, you need to give your employer notice "as soon as practicable." This translates into either 24-48 hour notice prior to taking leave, or within two business days of your return to work. As long as you were eligible for FMLA leave, you cannot be denied leave, in advance or in retrospect, for failing to give your employer sufficient notice.