Births by day of week: a historical perspective.

Our objective was to describe the historical pattern of the decline in weekend births. Data on 906,100 health maintenance organization enrollees' birthdates were analyzed to show patterns of birth by day of week from 1910 to 1999. The decline in Sunday births dates to the 1930s, and the decline in Saturday births dates to the 1950s, far earlier than previously demonstrated in the literature. The expected natural birth process has been significantly modified. By examining a much longer time series than in other literature, it is also possible to see that the trend is not abating and may be increasing.