The distinctive characteristics of the hourly distribution of live births on specific days in Japan.

OBJECTIVES: Anomalous variations in live births on February 29/March 1, April 1/April 2 and the days before the New Year holidays/New Year holidays have been reported in Japan. The distribution of live births was investigated on those days and whether or not such dates were selected due to obstetric intervention is discussed. METHODS: Using a method similar to the [Formula: see text]-R control chart, anomalous variations in the hourly number of live births were detected. The number of unusual births was estimated. RESULTS: The number of live births at 13:00-16:59 hours was significantly higher from December 24 to 28 and significantly lower from December 29-January 3, February 29, and April 1, especially on weekdays. In hospitals, the increases from 9:00-12:59 and 13:00-16:59 hours from December 24 to 27 were approximately 10 and 25 %, respectively, of the expected births for those times in the mid-1980s; thereafter, the rates were 30 and 35 %. After 2000, the child births at 13:00-16:59 hours on February 29 and April 1 decreased by approximately 35 % in hospitals and clinics. The numbers of live births at 0:00-0:59 hours were significantly higher on March 1 and April 2 until 2001. CONCLUSION: Anomalous variations at 0:00-0:59 hours may be associated with fictitious reporting on birth certificates. Anomalous variations from 13:00 to 16:59 hours on weekdays suggest that many individuals may avoid obstetric intervention on February 29 and April 1 and that the number of higher-risk deliveries may significantly increase in the daytime on the days before the New Year holidays due to obstetric intervention for institutional reasons.