Two months before she gave birth to Jason she left work feeling on top of the world.

[Nonverbis]

Two months before she gave birth to Jason she left work feeling on top of the world.
(https://www.collinsdictionary.com/dictionary/english/on-top-of-the-world)

Could you help me understand why not "she had left work"? Two events in the past, but she left work earlier than she gave birth.

 

[5jj]

When the order of events is clear, as it is with 'before' and 'after', we often don't feel the need to use the past perfect.

 

[emsr2d2]

Two months before she gave birth to Jason she left work feeling on top of the world.

Could you help me understand why it's not "she had left work"? There are two events in the past, but she left work earlier than she gave birth.

In my opinion, it needs a comma after "Jason".

 

[jutfrank]

Another reason beside what 5jj said not to use the past perfect is when the clause expresses an action that the speaker/writer wants you to focus on. We use the past perfect to provide background information. If the information isn't meant to be background, then it usually won't be in the past perfect. This is exactly the same case as your policeman example a few days ago.