If it feels like you are wishing people a happy birthday more often than usual once September begins, it is not in your head. September is the most common birth month of the whole year, and one day stands above them all—September 9.
In 2016, statistics organization FiveThirtyEight and data journalist Matt Styles analyzed data on the average number of births per year from 1994 to 2014. In that 10-year-period, September tops the with nine out of the 10 most common birthdays falling on September. Based on this analysis, the ninth day of the ninth month is the most popular birthday.
The reasons behind this surge of September babies are likely couples spending more time indoors during the colder months of the year and holiday season. Some people tend to work less as the holidays approach, so there is more time to party-hop, drink a few more cocktails than usual, and the rest is clear. Pregnancies are supposed to last 38 weeks from conception, so babies born on September 9 would have likely been conceived on December 17.
| Date | Average Number of Daily Births |
| September 9 | 12,344 |
| September 19 | 12,285 |
| September 12 | 12,282 |
| September 17 | 12,201 |
| September 10 | 12,190 |
| September 20 | 12,162 |
| July 7 | 12,147 |
| September 15 | 12,126 |
| September 16 | 12,114 |
| September 18 | 12,112 |
Source: birthdayrank.com
For a September 19 birthday, conception date should technically be December 27. All of those September 22 to 26 babies are generally conceived around New Years Eve and New Years Day. December is also a romantic season in general. According to data collected from the wedding planning website The Knot, December is the most popular month to get engaged globally.
Some other hypotheses behind this surge in cold weather conceptions are that the shorter days may affect hormones and that sperm may be less potent in the warmer summer months.
As far as rare birthdays go, Leap Day (February 29) is the clear winner, but the rest of the rare birthdays are those on major holidays. This is likely because scheduled inductions and C-sections are avoided on days like Christmas (December 25) and Fourth of July.
To see how common your birthday is, plug it into birthdayrank.com.
