The research teams looked at almost 4, 500 elderly people and, during a 90 minute home interview, graded their level of depression using the same scale, giving them points for every depressive symptom.

BBC: Depression may boost heart risk

In one study, funded by the National Institute of Mental Health, only one-third of patients achieved remission of depressive symptoms, defined as becoming symptom-free.

CNN: Using downward dogs to treat depression

Although in some cases, dark thoughts can be a severe and debilitating symptom of a postpartum anxiety or depressive disorder, usually they are just an unpleasant part of normal adjustment.

CNN: When motherhood becomes a nightmare