Final answer:
The landmark study mentioned does not specify the percentage of women with PPD who were also diagnosed with bipolar disorder, but both PPD and bipolar disorder are recognized as significant mood disorders among U.S. mothers.
Step-by-step explanation:
The landmark study you are referring to has not provided a specific percentage of women diagnosed with bipolar disorder among the 21% of mothers diagnosed with postpartum depression (PPD). However, it is known that mood disorders including PPD and bipolar disorder are significant issues among U.S. mothers. Bipolar disorder, characterized by dramatic shifts in energy and mood, can sometimes be associated with depressive episodes and thus may be comorbid with PPD. To understand the prevalence and overlap between PPD and bipolar disorder, more specific studies would need to be referenced.