Final answer:
The Landmark Study of 10,000 US Mothers did not provide specific information on the percentage of women with a primary diagnosis of unipolar depression among those diagnosed with postpartum depression (PPD). PPD can occur with or without a pre-existing diagnosis of unipolar depression.
Step-by-step explanation:
The percentage of women with a primary diagnosis of unipolar depression among the 21% of US mothers diagnosed with postpartum depression (PPD) is not specifically mentioned in the provided references. However, it is important to note that PPD can occur with or without a pre-existing diagnosis of unipolar depression.
Unipolar depression refers to a subtype of major depressive disorder (MDD) characterized by persistent sadness, loss of interest or pleasure, and other symptoms. The Landmark Study of 10,000 US Mothers did not provide specific information about the percentage of women with a primary diagnosis of unipolar depression among those diagnosed with PPD.
If you need more information on the topic, I recommend consulting additional sources or studies that specifically focus on the comorbidity of unipolar depression and postpartum depression.