I don't know much about postpartum depression, but 10-20% of women are estimated to have clinical depression after giving birth. These women will have negative feelings towards their baby, sometimes wanting to hurt them: "when a woman with severe postpartum depression becomes suicidal, she may consider killing her infant and young children, not from anger, but from a desire not to abandon them." Most women never act on these thoughts, but some women, like Andrea Yates and Susan Smith, killed their children because of this condition. In fact, according to this article, over 200 women a year kill their own children in America (!!!).
"Hansel and Gretel"
Even after the average birth, it's common to experience some form of "baby blues," emotions or sadness that go away after time. But 10-20% is pretty high-anyone who knows 10 mothers most likely knows at least one person who experienced postpartum.
It's a pretty strange thing to try to wrap your mind around, especially for those of us who have never given birth. Even today, when we are aware it's a serious medical condition, it's hard to understand what these new mothers are going through. How much more so hundreds of years ago? The mothers going through this would have really seemed like monsters if they tried to explain their feelings.
"Snowdrop" (A Snow White variant)
So many fairy tales are about mothers acting violently towards their children-remember that in older versions, Snow White and Hansel and Gretel were abandoned by their birth mothers, not stepmothers; but many other fairy tales, like Cinderella, Juniper Tree, Wild Swans, etc., feature abusive and/or murderous mothers. Maybe some of these stories came out of/were considered helpful for the fathers and family members who witnessed a mother experiencing inexplicably violent wishes towards her baby? Or maybe the stories were ways that the mothers themselves could vicariously fulfill their own desires without actually acting on them?
I don't think I've ever read anything connecting postpartum to violent fairy tale mothers, although if it really occurs so often, people throughout history would have witnessed mothers they know going through this, and it makes sense that the stories would help the different people involved start to process through the experience.