Hi KeninHK,
I really want to make something very clear--that if you are dealing with true post-partum depression it will be a lot more than just "keeping mother's spirits up" after birth. Post-partum depression has an actual hormonal, physiological as well as psychological root. It's not just someone who is "down in the dumps" or "needs a bit of cheering up." Often those types of mindsets are more harmful than helpful and unfortunately they're all too common.
Let me share my experience. My son was born at the end of 2007 in the States. The birth was a bit traumatic as I lost a lot of blood and could have died. It was a natural childbirth with no pain killers or other drugs. It was my first birth--my mother never had any complications in childbirth and neither had any of my female relatives so there was no reason for me to expect I would either. Within two days after the birth I was an emotional basket-case. Not only was I physically in a lot of pain and anguish (my legs were so swollen I couldn't walk--from IV fluid, I couldn't stand up for several days after the birth due to blood loss, my resting heartrate was 145 beats/minute, my hips were out of place and I couldn't walk straight, my lower back was herniated, I had large amounts of bruising and many, many stitches) mentally I was losing it. My baby cried all day and night and I just couldn't cope. My husband is very mellow and just kept telling himself and me that things would be okay--that he was strong enough to support me. In the first 4 days of my sons life I slept a total of 6 hours and not all at once--in 1-2 hour segments. I was delirious and this was only the beginning of what I was going to go through.
Well, we came back to Hong Kong and I was going to give the Stay-at-Home-Mom thing a go. My husband was away 12+ hours/day working in Mainland China (crossing the border twice/day and traveling for many hours). I was left at home, very isolated with serious back problems and the "baby blues." I really wanted to die. It was all I could do in a day to roll out of bed (literally, as because of a herniated disc in my back I couldn't stand up straight--at points I could only crawl on my hands and knees across our hard tile floor) and tend to the baby. Breastfeeding was a nightmare and I felt like a complete failure. I felt like my life was over. At times I had fantasies of jumping out of windows. But, still, my husband kept telling himself that we would be okay.
Then, I left my husband. I reached a breaking point where I really couldn't cope anymore and I left for about 10 days. I left him with the baby and didn't really know if I'd ever come back. I was in a world of pain and misery with a very cloudy and depressed mind. When I did come back he got down on his knees and apologized to me saying, "I was wrong. I thought I was enough. I thought I was strong enough. I thought I could fix this problem and help you and fix you. I can't." He went out the very same day and found a local woman to come and be a helper for us. She would come at 10 am every morning and stay until 5 pm. What did I do during that time? I crawled into bed and slept. At least the baby was well-taken care of and safe. The only cure for what was wrong with me was lots and lots and lots of rest. I slept like this for about four months. Needless to say, I don't remember much of that time. It is sad but I know that if I hadn't had done that it the outcome could have been much worse for my child, myself and my husband.
So, the best thing you can do is know the signs of post-partum depression and do your best to be watchful and intervene as soon as you can. It's nothing to mess around with. It's not just a "touch of the baby blues"--left untreated it can be absolutely devastating and even deadly. It took me almost a full 2 years before I could say I was "recovered" from PPD.
As far as what you can "expect" with hormonal drops--you will not know until you cross that bridge. Post-partum depression doesn't always have "early warning signs"--a woman who you'd least expect to suffer from it often gets it the worst. So, basically, be cautious.
I agree that doing everything in your power to provide your wife with rest and support is probably the wisest decision you can make. Make sure she is well-fed, well listened-to and cared for. This still doesn't guarantee that everything will go off without a hitch. I pray that she doesn't have to go through the type of thing that I and many other women have experienced but if she does it is very, very common. It is nothing to be ashamed of and remember whatever you keep in darkness will have power over you so bring everything into the light and be ready to get the right type of people involved if needed (nurses, doctors, friends, psychologists, nutritionists etc.).
And the last thing I want to add--whatever you do, don't expect your wife to just "bounce back" from birth--and don't let her expect that from herself either. If that's how it happens, great. But the best plan is to take it extra easy after birth for as long as possible.
All the best to you!