It's a little bit of a trick question because postpartum depression can affect any new mom. But it's important to note that there are some populations that are at a higher risk for postpartum mood disorders in general. Here are some of those groups:

Women who have had health-oriented /physical challenges in their past:

Survivors of intimate partner violence;
Survivors of sexual assault, rape or childhood sexual assault*;
Women who have suffered / suffer from disordered eating behaviors;
Women with a history of depression;
Women with a history of bipolar disorder;

Women with the following specific social or economic challenges:

Women with limited family support;
Women who are poor or lower income;

Lastly, there are factors related to the baby that put some women at a higher risk:

Special needs babies;
Colicky babies;
Chronically ill babies or babies with life-threatening illnesses.

[Author, IBCLC and speaker Dr. Kathleen Kendall-Tackett talks about all of the above challenges and more in greater detail in her book, Depression in New Mothers. I highly recommend it if you are interested in reading more about postpartum mood disorders.]

I would also add women who have suffered from chronic pain, after reading this article. This is just my opinion but I can't imagine how suffering from constant pain wouldn't make you predisposed to a postpartum mood disorder. Finally there is a new study out that says that women who fear childbirth are also at greater risk for postpartum mood disorders.

With all of these risk factors, I can't help but wonder why more women aren't diagnosed with a postpartum mood disorder. But of course, as we know, postpartum mood disorder screenings aren't thorough, perfect or even often enough. The onus is, once again, on the new mom herself to come forward. But when you're in a haze of confusion, pain, sleeplessness and perhaps anxiety, it can be hard to sort through whether your feelings are normal. Also not helpful is the fact that you don't see your OB until six weeks after you have your baby/babies. That may not be early enough (postpartum depression can happen anytime in the first year) or it may be too late.

Here are three things you can do, if you happen to fall into any of this categories of higher risk:

Make finding a good local new moms group, like mine, a priority for yourself before the baby arrives. Good social support is ESSENTIAL as a new mom. You will need to know that you aren't alone, that you (and your worries) are normal and that many new moms share your fears, wants and concerns. .
Consider hiring a postpartum doula. They have the distance and the expertise to recognize problems before they arise. Postpartum doulas are trained in, among other things, recognizing signs that mom gives...whether spoken or unspoken. Postpartum doulas aren't counselors but they can refer out to a qualified professional.
Speak up. If something is upsetting to you, talk with your partner, friend or another trusted mom about it. If you need something, ask for it. Too many of us suffer in silence, or just as damaging resentment, when it would be better for our physical and emotional health just to talk about it.

A postpartum mood disorder isn't a life sentence or something to be ashamed of. Let's keep the conversation going so we can all stay better informed and in doing so, be better advocates.