Whether there has been an increase in postpartum depression (PPD) over the generation remains unknown. This study aimed to compare the prevalence in two cohorts of young Australian women born 17 years apart and identified the factors associated with any generational differences. Participants were from the Australian Longitudinal Study on Women’s Health, who gave birth between ages 18 and 27 (born in 1973–78 and 1989–95). PPD prevalence was calculated as the percentage of births associated with PPD. Both the prevalence of PPD diagnoses (among 1,610 births) and PPD symptoms (among 953 births) were compared. Relative risks (RRs) and 95% confidence intervals (CIs) were used to report generational differences in the prevalence for PPD diagnoses; Hazard ratios (HRs) and 95% CIs used for PPD symptoms. Factors that differed between cohorts and were associated with PPD diagnoses or PPD symptoms were adjusted. The prevalence of both PPD diagnoses (21.4% vs 10.3%; crude RR: 2.03, 95% CI: 1.59–2.60) and symptoms (20.1% vs 13.3%; crude HR: 1.60, 95% CI: 1.15–2.34) were higher in the 1989–1995 cohort than the 1973–1978 cohort. Generational differences in PPD diagnoses persisted after controlling for potential contributors (RR: 1.53, 95% CI: 1.15–2.04), while generational differences in PPD symptoms were attenuated (HR: 0.98, 95% CI: 0.64–1.49). Of all contributing factors, a history of depression explained most of the generational differences, especially in PPD symptoms (49%), to the extent that when the study sample was stratified by history of depression, no generational differences were detected (without prior depression, HR: 0.65, 95% CI: 0.20–2.08; with prior depression, HR: 1.18, 95% CI: 0.71–1.96). The higher prevalence of PPD in the recent generation was mainly due to the high prevalence of depression. Strategies that well manage pre-existing depression may benefit the prevention of PPD for the current young generation. Further research is warranted to inform detailed prevention approaches.