Does the season of conception influence the frequency of post partum mental illness?

The study is based on two series of patients with post partum mental illness, one consisting of 71 women in Gothenburg 1952-56 and another of 68 women in Stockholm 1976-77. The total number of births in the two cities during the periods in question were used as reference. A strategy of chi-squared test and the Edwards' criteria of seasonality was applied for statistical analyses of the results. A yearly double periodic trend in month of conception could be observed.