A mother is in a vulnerable period during pregnancy and after child birth; her body shape and physiology are changing, her regular activity and social engagement is restricted during pregnancy.
Post birth, infant care is a challenging task as she recovers and regains her strength. Sleep is hard to come by as the infant needs to be fed.
While the mother is resilient, some factors can impact the mother's mental health adversely during this vulnerable period.
Prior history of depression or other mental health episodes, genetics
Stressful Life events before pregnancy
Marital Issues, in some cases Domestic violence or mental illness of partner
Societal pressures (child sex preference, skin colour etc)
Lack of family/social/community support or even conflict
Poor sleep habits
Lack of adequate or appropriate nutrition
Lack of Financial resources / Poverty
Untreated Childhood trauma / Sexual assault history
The important point here is that a number of these factors are beyond her control. One should not thus blame her for her low mood or anxiety which will only worsen her condition.This understanding needs to develop into empathy and compassionate support for the mother to find her resilience.
Here is a useful link for more detailed information