The estimates are a little higher than that as far as I am aware, up to 25% in some populations. To answer your question, it could be both - depending on the woman. I think, as with much mental illness, it is a combination of psychological, social, physical and life events. I'm not sure you'd be able to separate the two variables, as one can't exist without the other, and can't really be controlled for, because it would impossible to have a 'control group'.

I do know that women who have had abortions or miscarriages have been diagnosed with PPD, as have those who adopted their baby out. However, there are anecdotal reports from maternity providers that tell us that PPD can appear anytime from birth until the end of the first year of baby's life, even though the DSM might want us to categorise otherwise.

In my research (findings are still preliminary) I have found that 'it depends'. It depends if there is a prior history of mental illness, it depends if there are certain social conditions, it depends on the physical health, preparedness, maturity, and age of the mother, it depends on the kind of baby she has temperament-wise, it depends on her hormonal balance. Or maybe she has no risk factors, but is predisposed to it for other reasons that can't be identified readily? It is difficult to unpack all of the above to separate all factors out.

I would think this is a question that may never supply a clean answer - much like the nature/nurture debate.

Personally, I think the focus should be less on what the cause is and more about how to support and treat women, and have the right supports and services available in the community that will reduce the severity and duration of any symptoms she might have, and education around it, in order to reduce stigma. I read that while the estimates are between 10-25%, those estimates may be wildly incorrect, as only 50% of affected women will actually seek help.

More research is definitely needed in this area though, especially in the treatment for women with low to moderate symptomatology, as this is an area in which current systems worldwide are failing our women.

Conduct a study with 2 groups (minimum) of women. One group who bears and raises an infant. The second group adopts and raises an infant. The b/t group difference in this case would then be biological.

Some have suggested that neither child-bearing nor -rearing specifically serves as a causally agent, but instead post-partum depression could be related to social support the mother gets from others and how that affects the future prospects of both the child and the mother:

Hagen, E. H. (1999). The functions of postpartum depression. Evolution and Human Behavior, 20, 325–359.

The biological basis for PPD is well defined and also common genetic vulnerability with bipolar mood disorder was reported. Also rapid hormonal changes after delivery reported to be an biological stressor. So it seems that underlying biological susceptibility and pychosocial stressors are the main cause of PPD.

It is important to remember that post-partum depression, like other types of depression, is associated with many factors. Usually, many variables are in play and it is too simplistic to assume it is "caused" by any one event.

I have worked with many mothers in the post-partum period, having a baby is a life changing event; multifactorial. Bio psychosocial changes occur, society influences how woman adjust, family of origin issues a woman's adjustment, her relationship changes with herself as well her significant other, economic changes, plus hormonal changes, breast feeding etc., how was her delivery; was this a wanted pregnancy or not. Now she is responsible for a new life. I am sure it would overwhelm the best of us and YET.

It is interesting to read the responses, medical people say biological and mental health say it is the need of support. I think it is both.

Support makes a significant difference with readjusting. It is intriguing from my perspective that we do all this education for families, mothers before the child is born and then nothing. They go home and are pretty much on their own accept for doctors’ visits or if they are lucky they will have home care or family support.

You could develop an intervention, a new program started after delivery even if it was a group they called into on the phone for support once a week.

I think it would be interesting to compare the outcomes concerning postpartum depression with women who don’t have any intervention after they delivery and women who may have post-partum support classes.

I like Michael Schlund 's answer. My first thought was to look at depression rates of new fathers and siblings.

If I were to look for why PPD happens in women I would also (make this as simple and a least invasive as possible for the new mother.);

Collect socio-economic data and other important factors, such as breast feeding, types of help or spousal support, and a history of (happiness) interests.

Collect data monitoring Daily Hormonal levels and Neurotransmitter levels, like dopamine and serotonin. I think Saliva can be used for the neurotransmitters (which can be easily dated and mailed in... to lab.) so

Record sleep patterns, diet, weight and activities of the mother. Use a rating scale for mood. Twice a day (AM & PM) from delivery day to.....!

It could be interesting to collect same data from adoptive mothers and compare.

Experimental research may be answer. Given the fact that childrearing or child bearing are not causative of PPD in themselves but aggreviating or associated factors. Isolating child bearing from a bunch of other factors associated with PPD like, social support, finances, mode of delivery, how fast and easy the adoption process was etc, may not be easy. It's a big discussion!