[The effect of determining birthdays by social factors such as maternal preference on the distribution in the number of daily live births].

Objectives: To investigate the effect of determining birthdays by social factors on the distribution of daily live births.

Methods: We obtained data on live births tabulated by date and birthplace (hospitals, clinics, and maternity homes) between 1981 and 2010 in Japan from the National Vital Statistics System. This study analyzed the variations in live births for each day of the week, as well as for several specific days observed to have a systematic variation in the number of live births. We determined the ratio of the mean daily live births on those specific days to the overall daily average each month (the birth number ratio). The standardized deviation (the ratio of the deviation to the standard deviation for the day of week) regarding the birth number ratio of each specific day was also determined.

Results: The birth number ratio in hospitals and clinics was highest on Tuesdays and lowest on Sundays. Hospitals showed a large difference in the birth number ratio between weekdays and weekends, although the difference in the birth number ratio between weekdays and Saturdays was smaller in clinics than in hospitals. The birth number ratio during the first three days of the New Year was lower than that on Sundays. Until approximately 1995, the standardized deviation on February 29th and April 1st (the end of the Japanese school year) showed abnormally low values, while those on March 1st and April 2nd showed significantly high values. Following that time period, the significant variations on February 29th (only on Sundays), April 1st (only on Sundays), March 1st, and April 2nd almost completely disappeared. Maternity homes showed equivalent results until the 1980s or the middle of the 1990s.

Conclusion: 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.