structural:summarization, structural:expansion
Distribution through the year

Birthdays are fairly evenly distributed through the year, with some seasonal effects.

In the United States, there tend to be more births in September and October. This may be because there is a holiday season nine months before (the human gestation period is about nine months), or because the longest nights of the year also occur in the Northern Hemisphere nine months before. However, it appears the holidays have more of an effect on birth rates than the winter: New Zealand, a Southern Hemisphere country, has the same September and October peak with no corresponding peak in March and April. The least common birthdays tend to fall around public holidays, such as Christmas, New Year's Day and fixed-date holidays such as July 4 in the US.

In the United States between 1973 and 1999, September 16 is the most common birthday in the United States and December 25 the least common birthday (other than February 29, because of leap years). In 2011, October 5 and 6 were reported as the most frequently occurring birthdays.

In New Zealand, the most common birthday is September 29, and the least common birthday is December 25. The ten most common birthdays all fall within a thirteen-day period, between September 22 and October 4. The ten least common birthdays (other than February 29) are December 24–27, January 1–2, February 6, March 22, April 1 and April 25. This is based on all live births registered in New Zealand between 1980 and 2017.

Positive and negative associations with culturally significant dates may influence birth rates. The study shows a 5.3% decrease in spontaneous births and a 16.9% decrease in Caesarean births on Halloween, compared to dates occurring within one week before and one week after the October holiday. In contrast, on Valentine's Day there is a 3.6% increase in spontaneous births and a 12.1% increase in Caesarean births.

In Sweden 9.3% of the population is born in March and 7.3% in November when a uniform distribution would give 8.3%.