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While not all causes of PPD are known, several factors have been identified. Beck (2001) has conducted a meta-analysis of predictors of PPD. She found that the following 13 factors were significant predictors of PPD (effect size in parentheses -- larger values indicate larger effects):
* Prenatal depression, i.e., during pregnancy (.44 to .46)
* Low self esteem (.45 to.47)
* Childcare stress (.45 to .46)
* Prenatal anxiety (.41 to .45)
* Life stress (.38 to .40)
* Low social support (.36 to .41)
* Poor marital relationship (.38 to .39)
* History of previous depression (.38 to.39)
* Infant temperament problems/colic (.33 to .34)
* Maternity blues (.25 to .31)
* Single parent (.21 to .35)
* Low socioeconomic status (.19 to .22)
* Unplanned/unwanted pregnancy (.14 to .17)
Other factors identified in people suffering from postpartum depression are:
* history of genetic mental illnesses
* substance abuse
* former childbirth issues
These factors are known to correlate with PPD. That means that, for example, high levels of prenatal depression are associated with high levels of postpartum depression, and low levels of prenatal depression are associated with low levels of postpartum depression. But this does not mean the prenatal depression causes postpartum depression -- they might both be caused by some third factor. In contrast, some factors, such as lack of social support, almost certainly cause postpartum depression. (The causal role of lack of social support in PPD is strongly suggested by several studies, including O'Hara 1985, Field et al. 1985; and Gotlib et al. 1991.)
Although profound hormonal changes after childbirth are often claimed to cause PPD, there is little evidence that variation in pregnancy hormone levels is correlated with variation in PPD levels: Studies that have examined pregnancy hormone levels and PPD have usually failed to find a relationship (see Harris 1994; O'Hara 1995). Further, fathers, who are not undergoing profound hormonal changes, suffer PPD at relatively high rates (e.g., Goodman 2004). Finally, all mothers experience these hormonal changes, yet only about 10-15% suffer PPD. This does not mean, however, that hormones do not play a role in PPD. For example, found that, in women with a history of PPD , a hormone treatment simulating pregnancy and parturition caused these women to suffer mood symptoms. The same treatment, however, did not cause mood symptoms in women with no history of PPD. One interpretation of these results is that there is a subgroup of women who are vulnerable to hormone changes during pregnancy. Another interpretation is that simulating a pregnancy will trigger PPD in women who are vulnerable to PPD for any of the reasons indicated by Beck's meta-analysis (summarized above).
Women most commonly experience depression during their primary reproductive years (25 to 45), so they are especially vulnerable to developing depression during pregnancy and after childbirth.
Profound lifestyle changes brought about by caring for the infant are also frequently claimed to cause PPD, but, again, there is little evidence for this hypothesis. Mothers who have had several previous children without suffering PPD can nonetheless suffer it with their latest child (Nielsen Forman et al. 2000). Plus, most women experience profound lifestyle changes with their first pregnancy, yet most do not suffer PPD.
In severe cases, postpartum psychosis (also known as puerperal psychosis) can develop, characterized by hallucinations and delusions. This happens in about 0.1 - 0.2% of all women after having given birth. In some cases, postpartum psychosis can develop independent of postpartum depression.
Sometimes a pre-existing mental illness can be brought to the forefront through a postpartum depression. Postpartum depression can be hereditary. Women with severe premenstrual syndrome most commonly suffer from postpartum depression.
Evolutionary psychological hypothesis
Evolutionary approaches to parental care (e.g., Trivers 1972) suggest that parents (human and non-human) will not automatically invest in all offspring, and will reduce or eliminate investment in an offspring when the costs outweigh the benefits, that is, when the offspring is "unaffordable." Reduced care, abandonment, and killing of offspring have been documented in a wide range of species. In many bird species, for example, both pre- and post-hatching abandonment of broods is common (Ackerman et al. 2003; Cezilly 1993; Gendron and Clark 2000).
Human infants require an extraordinary degree of parental care. Lack of support from fathers and/or other family member will increase the costs borne by mothers, whereas infant health problems will reduce the evolutionary benefits to be gained (Hagen 1999). If ancestral mothers did not receive enough support from fathers or other family members, they may not have been able to afford raising the new infant without harming any existing children, or damaging their own health (nursing depletes mothers' nutritional stores, placing the health of poorly nourished women in jeopardy).
For mothers suffering inadequate social support or other costly and stressful circumstances, negative emotions directed towards a new infant could serve an important evolved function by causing the mother to reduce her investment in an unaffordable infant, thereby reducing her costs. Numerous studies support the correlation between postpartum depression and lack of social support or other childcare stressors (Beck 2001; Hagen 1999).
Mothers with postpartum depression can unconsciously exhibit fewer positive emotions and more negative emotions toward their children, are less responsive and less sensitive to infant cues, less emotionally available, have a less successful maternal role attainment, and have infants that are less securely attached; and in more extreme cases, some women may have thoughts of harming their children (Beck 1995, 1996b; Cohn et al. 1990, 1991; Field et al. 1985; Fowles 1996; Hoffman and Drotar 1991; Jennings et al. 1999; Murray 1991; Murray and Cooper 1996). In other words, most mothers with PPD are suffering some kind of cost, like inadequate social support, and consequently are mothering less.
On this view, mothers with PPD do not have a mental illness, but instead cannot afford to take care of the new infant without more social support, more resources, etc. Treatment should therefore focus on helping mothers get what they need. (See Hagen 1999 and Hagen and Barrett, n.d.)
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