I only have one child and no experience/historty of PPD so I can't say that consuming my placenta prevented it, but I do know that it helped with a postpartum hemorrhage. My midwife's assistant mixed part of the placenta into a berry smoothie about an hour after DS was born. At that point, I was bleeding pretty heavily, had been given a shot of Pitocin (I believe?) that didn't seem to be working, and the midwife was beginning to consider a transfer to the hospital (it was a homebirth.) Once I had the smoothie, the bleeding began to stop within 15 minutes, and an hour later, it had stopped all together. I don't know if the two are related, but I do know that I felt great after my birth-- no PPD, good energy and spirits. I consumed the rest of the placenta raw in smoothies over the next 2 or 3 days-- a dear friend was kind enough to chop/prepare it for me because I was pretty squeamish :/
I hope you get some good data from this thread-- thanks for starting it!