There are 3 levels of depression that follow birth. Post-partem blues, which is the most mild and is shown by bouts of sadness or lethargy. Post-partem depression, which I had, is the second and in addition to the other symptoms, one can also feel overwhelming depression and harbor thoughts of either harming the baby or that others might. Post-partem psychosis is the most serious. Bouts of delusions can occur and the mother could harm herself or others if not treated.
http://www.4woman.gov/faq/postpartum.htm
Your wife needs to be open with her doctor about her feelings and her daily routine so that she can be properly assessed and helped. Good luck.