Background: Postpartum depression (PPD) is a widespread worldwide phenomenon, but its etiology remains unclear. This study reappraised how evolutionary theory could explain PPD as an adaptation through investigating the relationship between maternal age and PPD, and if this relationship is modified according to the number of children at home. Methods: A cross-sectional study carried out in five primary health care units included 811 participants randomly selected among mothers of children up to five postpartum months in Rio de Janeiro, Brazil. Postpartum depression was defined by scores above 11 on Edinburgh Postnatal Depression Scale (EPDS), and statistical analysis was based on multivariate logistic regression models. Results: One hundred and ninety-seven (24.3%, CI 95% 21.3–27.2) participants were classified as PPD positives. Maternal age was significantly associated to PPD (OR¼0.96, p-value¼0.019) independently of socioeconomic and reproductive characteristics, conjugal status or substance consumption by the couple. Thus, for each additional year, a reduction of 4% in the chance of developing PPD could be anticipated, effect which was not modified by the number of children at home (p-value¼0.602). Limitations: Information on social support was not included in this analysis since its relationship with maternal mental health would be better evaluated in a prospective fashion. Conclusions: These findings suggest that adaptive mechanisms shaped through human generations persist contributing to the development of PPD in contemporary societies. According to this evolutionary approach, as maternal age advances the reproductive potential diminishes and, consequently, mothers are less prone to develop PPD and reduce investment in new offspring.