September 9 is the most common birthday in the United States.

Data collected by the U.S. National Center for Health Statistics and the Social Security Administration reveals a consistent trend in birth patterns over several decades. According to a study by Harvard University researcher Amitabh Chandra, September 9 is the most frequent birth date for Americans born between 1973 and 1999. On average, approximately 12,301 babies are born on this day each year in the United States.This seasonal spike is largely attributed to the holiday season. Counting back 40 weeks from mid-September lands squarely in December and early January. During these colder months, couples spend more time indoors, and the festive atmosphere of Christmas and New Year's often leads to a higher rate of conception.Interestingly, the least common birthdays are almost all holidays. December 25, January 1, and July 4 consistently rank at the bottom of the list. This is primarily because doctors and parents rarely schedule elective C-sections or induced labors on public holidays, leading to a significant dip in hospital births during those 24-hour periods.While September 9 holds the top spot, the entire month of September dominates the rankings. Nine of the top ten most common birthdays fall between September 9 and September 20. This biological and social phenomenon highlights how environmental factors and cultural celebrations directly influence human reproductive cycles on a national scale.