This study examined a sample of preschool-age children with autism in an attempt to identify patterns of birth dates that deviated from expected frequencies by month or season. Birth dates of children with autism and those of a non-autistic sibling control group were compared to the number of total live births gathered from U.S. Census data. Analyses included two types of chi-square analyses and a seasonal harmonic trend analysis. Previously unmentioned in the literature is a seasonal effect finding for females within the entire sample, and both a seasonal and monthly effect for children classified as socially Passive by the Wing system. A significant elevation was also found in March within the Boston sub-sample (n = 37). This sample largely comprised low-functioning boys with autism, a finding consistent with previous findings in the literature. Peri-natal complications and early life development of the subjects from the Boston site are detailed.