Background: Much has been written about postnatal depression as a clinical condition. There is some evidence to suggest that a substantial proportion of women who give birth experience a depression in the postnatal period. This paper reports the results of a longitudinal study of the mental health of a large sample of women who were in the early stages of pregnancy at entry to the study. Methods: Each participant was assessed for symptoms of depression at the first clinic visit (entry to the study), and reassessed at various intervals – at 3–5 days, at 6 months, and again at 5 years after the birth of the child – using the DSSI-D (Delusions-Symptoms-States Inventory). Results: Retrospective recall questions indicate that shortly after the birth the majority of women experienced some depressed mood. Of those who experienced depressed mood, the data suggest that the symptoms were not severe, nor did these symptoms generally continue beyond a few weeks. The longitudinal data indicate that levels of depression in our sample are highest either at the first clinic visit or at the 5-year follow-up. Rates of depression at the 6-month follow-up are relatively low by comparison. Conclusion: While most mothers experience periods of depressed mood after the birth of their baby, these periods are generally of short duration and of lesser intensity than a major depression. Mothers appear to experience increasing levels of symptoms of depression as their child grows up. Many of the “cases” of depression experienced at the 5-year follow-up represent a recurrence of a previous experience of depression.