Talk to your obstetrician about this. You can have hormone therapy post-natally that has had some good results.
De Katherina Dalton believes that keeping your blood sugar stable can help prevent PND, because I your blood sugar drops, then the receptors which should pick up the hormones from your blood stream, switch to picking up glucose from your blood stream instead (to maximise the amount of energy your body can access). She recommends eating some carbs no more than an hour after getting up, three hourly through the day, and no more than an hour before you go to sleep.
You don't have to eat lots more carbs, just little and often. I heard her lecture on this when I had PND (and I have to be honest and say I had it after each of my three dc, but in my case it was complicated by, as then undiagnosed! clinical depression). What I liked about this approach is that it is treatment without drugs, and puts you in control.
It is also worth talking to your family - as this has happened once, you and they will know how it manifested in you last time, so can be alert to any early signs that you are developing it again, so you can treat it early if it happens.
One thing that I hope might set your mind at rest a bit - as I said, I have had PND thre times, and have had clinical depression since I was 14 (undiagnosed until I was in my 40s), and my boys have grown up healthy, happy, stable and successful.