Final answer:
The percentage of women who endorsed thoughts of harming themselves among the 21% of mothers diagnosed with postpartum depression (PPD) in the Landmark Study of 10,000 US Mothers is not provided.
Step-by-step explanation:
The percentage of women who endorsed thoughts of harming themselves among the 21% of mothers diagnosed with postpartum depression (PPD) in the Landmark Study of 10,000 US Mothers is not provided in the information you provided. The reference you provided, Wisner et al. 2013, does not contain this specific information. Therefore, without further information, it is not possible to determine the exact percentage of women who endorsed thoughts of harming themselves.