Characteristics of the distribution of the hourly number of live births on specific days from 1981 to 2011 in Japan
In a study of Chinese births, it was found that socio-cultures reasons were a common indication for Caesarean delivery, such as that the mother wished to give birth on a date believed to be particularly auspicious (Lei, 2003). In a study of Japanese birth statistics, it was shown that there was an abnormal variation of live births on 2/29 and 3/1 and 4/1 and 4/2. These phenomena indicated that some birthdays were based on maternal preferences (the day after leap day, the day starting the next school year). The purpose of this study was to investigate the distribution of live births by specific days, and to discuss whether the dates were set using obstetric intervention.
METHODS:
The data on live births tabulated by time (one-hour intervals), date and birthplace (hospitals, clinics) between 1981 and 2011 in Japan were obtained from the National Vital Statistics. The ratio of the hourly live births to the mean of the overall days of the month (hourly birth number ratio) was determined. The average and standard deviation of the hourly birth number ratio for each day of the week in each year was determined. The deviation standardized for the hourly birth number ratio of each specific day was also determined.
RESULTS:
The standardized deviations on 2/29 and 4/1 showed abnormally low values (about -3 to -7) from 13:00-16:59. These characteristics were always observed on weekdays, namely, from Monday to Friday. The standardized deviations on 3/1 and 4/2 showed abnormally high values (about 3 to 11) from 0:00-0:59 until 1995 in hospitals and until 2001 in clinics, regardless of the day of the week.
CONCLUSIONS:
The abnormal variations at 0:00-0:59 may be associated with factitious reporting on birth certificates. More recent variations on 2/29 and 4/1 suggest that many individuals may avoid obstetric intervention.