When we hear "depression" we often assume that means lethargic, apathetic, tired, and detached. In reality postpartum depression can manifest itself in a variety of ways - aggression, irritability, anger, confusion, etc. This can be directed towards a spouse, the baby, older children, or the mom herself. Postpartum depression is a very REAL problem, and one that needs to be addressed for the benefit of the entire family.
This is the Edinburgh Scale to assess postpartum depression. Please discuss this with your partner, care provider, and doula - Edinburgh Scale
This is the Postpartum Promise, another way that moms can let their care team know in advance that she's aware of PPD and would like their help in assessing her after birth - PPD Promise
In addition to postpartum depression there are other mood disorders - the less severe "baby blues", postpartum anxiety, and the very serious postpartum psychosis. Every mother should be screened during her pregnancy for a history of depression, anxiety, or other mental health concerns. Care providers should also be checking on moms during postpartum visits, but often an OB doesn't seem a mom again until six weeks after the birth. (Midwives generally see the mom sooner - 3 days, 2 weeks, and 6 weeks postpartum.) Partners must be aware of these concerns as they are the closest to mom and will be the first to notice something is amiss. This is an important discussion to start DURING pregnancy - I include questions in my intake paperwork and ask about it further during prenatal visits, then follow up in postpartum visits. As a mom who experienced severe postpartum depression I want to ensure that no mom feels alone in this struggle. As doulas it's important that we also be checking on moms to make sure they receive the care they need.
Here are two excellent books to learn more about it.