My  suggestion is that the evolution of subcutaneous fat in neonates and post-natal depression is an example of co-evolution and what has happened is mothers are programmed to be depressed for the first few days after birth because in primal conditions when remember this is where evolution set the parameters of human behaviour, in primal conditions it would probably pay a mother to test her new-born offspring to see if it could survive, because if it was defective or if it was sickly, or if there was some reason why that baby couldn't make it to adulthood and its own reproductive life, that mother should not invest in it, because that's her reproductive success.