Recent research has revealed a surprising link between inflammation and depression. Although inflammation was first identified as a risk factor for postpartum depression in the late 1990s, researchers have recently learned that it underlies all the other risk factors. All other known causes of depression, such as pain, psychological trauma, lack of social support, or sleep problems, trigger the inflammatory response. This research tells us why women are vulnerable to depression in the last trimester of pregnancy and postpartum. It also explains the link between depression in pregnancy and preterm birth, and why breastfeeding protects maternal mood because it downregulates the stress and inflammatory response.