A study conducted by investigators from the National Institutes of Health in Bethesda, has shown that feelings of stress or depression during pregnancy are associated with changes in the placenta. 

The findings, published in Epigenomics, suggest that these changes could modify gene activity. Investigators assessed 301 pregnant women from 12 clinics in the United States who had participated in an earlier clinical study. The participants completed questionnaires used to screen for stress and depression at the start of study and five more times during pregnancy. After delivery, tissue samples were taken from the placenta to analyze the genetics. In 16 areas, changes to the exterior of placental DNA were found to be associated with depression in the second or third trimester. Additionally, two areas were identified where these changes were tied to stress in the third trimester… (Medscape)