In countries close to the equator November is the month with the most birthdays and those furthest from the equator have the most birthdays in September. This seasonal pulse can also be seen within the Northern and Southern United States, with northern babies being born earlier in the year.
Humans tend to have babies throughout the year, rather than in certain seasons like other species. However, there are certain months when many more babies are born than usual. These birthday months can vary from country to country depending on how close one is to the equator. Slate.com states that research has shown those countries which are closest to the equator and tend to stay warmer year-round have more birthdays in November overall. However, those countries which are further away from the equator tend to have the most birthdays in September.
The United States happens to have the most birthdays in the months of August and September, depending on how many previous years are taken into account. According to LiveScience, as the year progresses the number of birthdays each month gradually increases until late summer, then it begins dropping until February, which is the month with the least number of birthdays. It should be noted that overall the numbers of births per month vary only a little between about 319,000 in February and about 387,000 births in August on average.