September was the most popular birth month in the United States in 2010, and data taken from U.S. births between 1973 and 1999 indicates that September consistently has the densest cluster of common birthdays. The same data from 1973 through 1999 indicates that January has the smallest number of births.
The most common birthday in the United States is September 16. All of the top 10 most common birthdays are in September, and birth rates are very high from September 8 until October 2. Birth rates in general are much higher from June until mid-October. January has much fewer births than other months, even compared to the fewer number of births in December and February. There is also a consistent decline in births on the 13th of every month, perhaps due to superstition. The three least common birthdays are February 29, December 25 and January 1.