Final answer:
The specific percentage of women whose depressive episodes began during the postpartum period within the 21% diagnosed with PPD from the Landmark Study cannot be determined with the given information. According to Wisner et al. (2013), depressive symptoms were identified in women during the postpartum period.
Step-by-step explanation:
The question pertains to a finding within the Landmark Study of 10,000 US Mothers, specifically regarding the percentage of women diagnosed with postpartum depression (PPD) whose depressive episodes began during the postpartum period. Unfortunately, the data provided does not contain enough information to accurately respond to the question about the specific percentage of the onset of depressive episodes during the postpartum period for those diagnosed with PPD. Therefore, without access to the complete study or proper data, it is irresponsible to provide a specific percentage. However, according to a study by Wisner et al. (2013) referenced in the provided information, women with screen-positive depression findings did exhibit symptoms of depression within the postpartum period.