What the field actually says about this period
Two overviews of the same territory. The first sets out what these states are and how they are told apart. The second pooled the studies on how common low mood is across pregnancy and the year after, and it is worth reading its own warning as carefully as its result.
What the research says
O'Hara MW, Wisner KL (2014). Perinatal mental illness: definition, description and aetiology. Best Practice and Research Clinical Obstetrics and Gynaecology, 28(1), 3 to 12 doi
Sets out the three states described in lesson one as separate things rather than as points on one scale, and describes the biological, psychological and social contributions that converge after a birth.
Woody CA, Ferrari AJ, Siskind DJ, Whiteford HA, Harris MG (2017). A systematic review and meta-regression of the prevalence and incidence of perinatal depression. Journal of Affective Disorders, 219, 86 to 92 doi
Pooling studies from many countries, depression across pregnancy and the year after birth is common rather than unusual, and is found more often in lower and middle income countries. The authors flag that the studies varied a great deal from each other and that most relied on questionnaires rather than on a diagnostic interview, so the pooled figure should be read as an estimate and not as a count.