This study aimed to identify trends and factors in postpartum depression after the Fukushima nuclear accident among women who participated in the Fukushima Health Management Survey (FHMS). We analyzed the prevalence of depressive symptoms and concerns about radiation exposure using FHMS data from FY2011-FY2018. We also analyzed the characteristics of women who did not have depressive symptoms in the postpartum period but had developed them at 4 years after childbirth and women with depressive symptoms in the postpartum period but had improved at 4 years after childbirth. In this survey, depressive symptoms were evident in 23.1% of women in the postpartum period. Factors significantly associated with developing depressive symptoms at 4 years after childbirth were younger maternal age, not being healthy, lacking maternal confidence, history of mental illness, and not intending to become pregnant again because of radiation effects. Improvement from initial depressive symptoms at 4 years after childbirth was associated with being very healthy, having maternal confidence, no medical history, no history of mental illness, having received sufficient perinatal care, and intending to become pregnant again. Our results suggest the need for comprehensive assessment of depression risks for prevention and thorough continuous professional care for disaster affected women with postpartum depression.