M3lon8 is correct according NHS pages.
www.nhs.uk/conditions/post-natal-depression/
The cause of postnatal depression isn't completely clear. Some of the factors it has been associated with include:
a history of mental health problems, particularly depression, earlier in life
a history of mental health problems during pregnancy
having no close family or friends to support you
a poor relationship with your partner
recent stressful life events, such as a bereavement
experiencing the "baby blues"
Even if you don't have any of these symptoms, having a baby is a life-changing event that can sometimes trigger depression.
Under myths same page
Postnatal depression is entirely caused by hormonal changes. It's actually caused by many different factors.
Postnatal depression will soon pass. Unlike the "baby blues", postnatal depression can persist for months if left untreated. In a minority of cases, it can become a long-term problem.
Postnatal depression only affects women. Research has actually found that up to 1 in 25 new fathers become depressed after having a baby.
I'm not convinced lumbering men with pnd rather than a new label is helpful but NHS is apparently on board with it.
As for what to do - better screening for men with accompanying more awarness with GP, more support for both parents, more general awareness of impact of babies? I don't know - I still think research into causes and effective preventatives or treatments should be in there as well.