Unfortunately, post-natal depression is still prevalent in women. It was regarded as a draconian myth many years ago because people assumed women were simply too happy with the birth of their child to have time for sadness. That is not, however, the case.
I have witnessed friends as well as family members suffer with post-natal depression, and mostly it is due to a lack of support from people around them. They are alone with their newborn most of the day while their partner works, but, from what I've seen, it's the inability to lead the life you did previous to giving birth. Whether a baby is planned or not, life changes dramatically when they arrive, so in reality, life isn't all rosy once a baby is born. From what I have witnessed, however, post-natal depression does subside and eventually 'go away', so women can finally take motherhood in their stride, and enjoy their little newborn.