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Peripartum depression affects around 12% of women in pregnancy and postpartum, and about 2–3% of European pregnant women use antidepressants, mostly selective serotonin reuptake inhibitors (SSRIs). An increased risk of poor pregnancy outcomes has been described in women with antenatal depression and SSRI treatment during pregnancy. The biological mechanisms behind these complications are not fully understood and here we investigated several biological correlates of peripartum depression, and discriminated between the effects of antidepressant treatment and depression itself.

In Paper I, attentional biases in pregnant and postpartum women were studied by using the Emotional Stroop Task, measuring reaction times to different stimuli. The major finding was shorter reaction times in postpartum depressed women, for emotionally valenced stimuli, which can be interpreted as emotional numbing.

In Paper II, peripheral inflammatory markers were assessed by proximity extension assay technology in depressed, SSRI-treated and healthy pregnant women. Lower levels of 23 markers were found in women with antenatal depression, independent of treatment, compared with healthy controls. These findings suggest a dysregulated switch to the anti-inflammatory M2 milieu characterizing a normal third trimester.

In Paper III, normal changes in inflammatory markers across pregnancy and postpartum were assessed in healthy pregnant and postpartum women. The majority (41) of the 50 markers that differed between groups were lower postpartum. These results clearly reflect the change in the immune system in pregnancy to postpartum transition.

In Paper IV, placental gene and protein expression were investigated and nominally significant findings were noted for serotonin receptor 1A (HTR1A) and neuropeptide Y2 receptor (NPY2R), where women with untreated depression displayed higher gene expression than healthy controls. Protein expression analyses revealed higher levels of HTR1A in placentas from SSRI-treated women, compared with healthy controls and women with untreated depression. This suggests possible involvement of HTR1A in the effect of antenatal depression on the placenta.

Overall, peripartum depression is associated with altered cognitive-emotional processing, lower levels of several mostly anti-inflammatory markers, and altered placental gene and protein expression. However, we found no major differences between untreated and treated depression.