The variations in the days of the week were inconsistent with nationwide policies for consultation in each setting. These results indicate that some birth dates were set for institutional reasons or maternal preferences (i.e., the day after the leap day or the next school year) by using or avoiding obstetric intervention. The abnormal variation on leap days and on April 1st might be related to fictitious reporting. More recent variations in the birth number ratio on specific days suggest that some individuals may avoid obstetric intervention. The results of this study indicate that determining birthdays by social factors may have been practiced in maternity homes until approximately 1990.