I'm wondering if the real origin of the use of the term "depression" stems from the fact that "they" know the brain is literally depressed and not just referring to your emotional state. I don't believe we are being told the truth
My understanding is that it's like when the weather person on TV says 'we're currently in a depression right now'; which means "L"; and both "L" and "H" are comparitively of long duration (usually at least a few days). On the other hand, a thunder storm, hail storm, torrential rain ... these are all short-duration things typically.
So in terms of depression (which basically is having a low mood that lingers), if one has low-level depression (eg dysthymia), then it lasts indefinately, and perhaps is a permanent issue. If it's major depression (eg MDD), then it also lasts a fair while. But one can also just be "depressed" about something, which might only last a day or so, and which i regard as being different to "depression".
And connecting this back to birthing trauma, i believe both physical & psychological traumas at (& before) birth, can possibly make the person much more susceptible to mental disorders such as GAD, depression, ADHD, SA, ...