T cells play a key role in adaptive immune responses, and shifts among T cell classes occur in normal pregnancy. There is evidence for the role of TH17 cells and dysregulation of the TH17/Treg cell balance in morbidities and autoimmune diseases during pregnancy. Because TH17 responses may play a role in depression and anxiety outside of pregnancy, we hypothesize that TH17 responses and the balance of TH17/Treg activity may also contribute to the development of depression and anxiety during pregnancy. To explore this hypothesis, this review has three main aims: 1) to evaluate systematically the role of TH17 cells and cytokines during pregnancy; 2) to compare changes in the ratio of TH17/Treg cells during pregnancy morbidities with the changes that occur in depression and anxiety outside of pregnancy; and 3) to provide a basis for further research on TH17 cells in perinatal mood and anxiety disorders, with an eye toward the development of novel therapeutics. We also review the limited literature concerning perinatal mood and anxiety disorders, and hypothesize about the potential role of TH17 cells in these illnesses. Understanding the pathophysiology of perinatal mood and anxiety disorders will aid development of novel therapeutics that address immunological mechanisms, in addition to the serotonin system, which are targetable molecules in treating depression and anxiety during pregnancy.