At the time I was born (and I was a planned baby; her first. I arrived almost two years after my parents were married) my mother had undiagnosed borderline personality disorder. She was a month away from turning 23 when I was born. From what I've been able to gather, my mother had a "nervous breakdown" after they brought me home from the hospital and was unable to care for me. So my dad asked his mother if she could come and stay with them for a few weeks to show my mother how to care for an infant. (My mother's mother and father had gone on an extended vacation at around the time I was due.) I believe that my mother went beyond post-partum depression, into post-partum psychosis. Mother told me decades later that she had been convinced that I as an infant, a newborn, had hated her and rejected her as a mother, and she maintained that fixed delusion that I was a cold and unloving, rejecting child for the rest of her life.
As you can imagine, my mother's fixed delusional belief had a terribly negative impact on my emotional development. Although mother had no such bizarre beliefs about my younger sister, mother still subjected both of us to extreme, frequent emotional and physical abuse throughout our growing up years.
So, I can see how studies would show that post-partum depression and even post-partum psychosis might occur more frequently in women who already have a psychiatric condition like a mood disorder, a psychotic disorder or a personality disorder.
I understand that in England, there is a program in place that sends visiting nurses to new mothers at various times throughout the first year or two of the child's life to assess whether the child is thriving or not, get a feel for whether anything is off-kilter in the home, the parent's feelings and behaviors regarding the child, if the child seems to be securely attached or not, etc.
I wish we had a similar program here in the States, for all new mothers. And, seems to me, the more children a woman has, the more need there would be for reassessments with each new birth.
Anyway, its great that you are bringing attention to this important subject. Thank you.