Why September 9 is the most common birthday

US birth data shows that September is the most common birth month and September 9 is the most common birthday. Winter holiday conceptions appear to be the main explanation, although seasonal biological factors may also play a role.

美国出生数据表明，9月是最常见的出生月份，而9月9日是最常见的生日。冬季假日期间受孕似乎是主要原因，不过季节性的生理因素也可能产生影响。