The signs would be there, though. As every mother finds out in short order, "motherly instinct" is over-rated, and can't be relied on to instruct one on how to properly change a diaper, among most all other things. However, it is extremely powerful in "protect at all costs" - in humans, birds, dogs, pigs, etc.,. I feel that any disorder or even environmental conditions that could over-ride that instinct would be detectable.
When I came home and told my husband I had PPD he looked at me and said "what are you talking about, your fine". Rose tinted glasses much hubby?? The signs were there, but his brain chose to not see them because he didn't want there to be a problem. I was living in a house with him, his parents, 2 brothers and a sister and not a single one of them thought I had a problem. Due to the nature of "mental health" and it being a bit of a "under the carpet" topic, quite often people don't want to believe there is an issue. I clearly remember standing there with my young baby who was screaming, rocking her, with tears running down my face and his brother and the brother's girlfriend walking in, looking at me saying hi and then making themselves a cup of tea and leaving me to deal with it.
Just today when leaving the inlaws my sis in law (who is 8 months preg, has a 1 1/2 yo and a 3 yo) was in tears and told me she is depressed. I went in and told her mother to give her a call and have a chat with her. The response? Oh she's just tired, she'll be fine. This girl a few months ago said she was depressed but I may as well be banging my head against the wall, because convincing people that depression isn't about having a bad day.
There is still a stigma of sweeping it under the rug, it's not acceptable by many to admit to these issues.
Golden Horse - So sorry to hear about your nephew, and I'm glad years ago you were able to find the help you needed. I can relate all too well, I was over here with no family or good friends.. I had only my husband and his family and felt like a total outcast. To this day I struggle with the lack of support as I haven't gelled with many Irish to a degree that I would consider them good friends.. plenty of acquaintences but nobody I could speak to and feel trust in.