A post-mortem is performed by a Doctor to determined how someone died.

It's usually after the event and it may uncover that an artery was clogged leading to the stroke and ultimately death.

A pre-mortem is the opportunity to think about what may happen and preemptively working to avoid or fix a situation.

In the stroke example, if you are concern about your heart health you may decide to take measures to be healthy.

Take inventory of what's important to you and perform a pre-mortem. 

It's free and and help you before it's too late!