Postpartum disorders include a spectrum of psychopathology including the postpartum blues, postpartum depression, and postpartum psychosis. Brief episodes of hypomania are quite common immediately after delivery but are rarely diagnosed. There is substantial evidence that postpartum psychosis is usually a variant of bipolar disorder in the form of a mixed or manic episode. Research studies however, have neglected the clinical reality that the postpartum period is also a high-risk time for the occurrence of episodes of bipolar depression. Diagnosing bipolar depression is not difficult in women with a history of a mixed or manic episode. However, misdiagnosis of bipolar II depression may be common after childbirth due to the likelihood that hypomania may be misconstrued as the normal joy related to the experience of motherhood. Early and accurate recognition of bipolar disorder is crucial as the use of antidepressants in patients with a bipolar diathesis can worsen the illness course due to induction of hypomanic, manic, and mixed episodes as well as acceleration of cycle frequency. This paper reviews the relationship between postpartum depression and bipolar disorder, and discusses the clinical and treatment implications of misdiagnosis of bipolar II depression as an episode of major depression.