Happy Birthday!

Happy Birthday!

The chances are high that many people you know have a birthday this month. That is because September has more births than any other month---by a significant margin. 

This was the conclusion of research conducted by Amitabh Chandra of Harvard University who tabulated every birth in the United States from 1973 to 1999. You can see where your birthday ranks in that study from this chart from The New York Times which I wrote about previously in 2012.

The original study has been subsequently updated with birthdates from 1994 to 2014 (by the number crunchers at FiveThirtyEight) and Matt Stiles of The Daily Viz developed a heat map graphic to display the data.

How common are September birthdays?

Nine out of the top ten most common birthdays are in September. The only non-September birthday to crack the top ten list was July 7.

September 9 was the most common birthday in the most recent study. It was 2nd in the previous study. September 16 was the most common birthday in the Harvard study. It was 9th in the new study.

The least common?

December 25. That is followed by these dates as the least common birthdates. 

December 24. January 1. July 4. 

It appears that something beyond nature is going on here. It would be interesting to see C-section rates just before these dates. For example, December 20 is the 11th most common birthday (have the baby and be home for Christmas?). December 29 is 15th (get the tax exemption and be home for New Year's Eve).

February 29 was not the least common birthday in the new study despite the fact that it only comes around every four years. This was due to the fact that the statisticians used average births per year in developing the data. It was 347/366 overall by this calculation.

January has the fewest births of any month. No date in January gets higher than 199th---January 24th.

In fact, 15 January birthdays fall in the 60 least common birthdates. Not a good month to be in the birthday cake business.

Similar birthdate patterns are found in the United Kingdom where that country's Office of National Statistics did a similar study of birthdates in England and Wales involving births from 1995-2004.

The UK study found September 26 to be the most common birthday (that date was 12th in the U.S.). The least common---December 25 followed by December 26 (Boxing Day in the UK is also a  big public holiday.)

Credit: UK Daily Telegraph

This is a heat map of UK births where you can see the similarities to the U.S. data. above.

Why are more births clustered in July through early October?

Why are there proportionally more babies born in and around September?

Have you considered the fact that September is nine months after December for starters?

A study in Obstetrics and Gynecology  in 2001 reached the following conclusions for why there seem to be more conceptions in the months from November through January than other periods of the year.

Biologic processes or common behaviors may account for the seasonal variation. Biologic hypotheses include deterioration of sperm quality during summer, seasonal differences in anterior pituitary-ovarian function caused by changes in the daylight length, and variation in quality of the ovum or endometrial receptivity. Increased sexual activity associated with end-of-year holiday festivities has also been postulated as a possible behavioral explanation for the December peak in conceptions. The exact reasons remain unknown.

Is it holidays, daylight length or cold nights? It seems nobody knows for sure. Even the OB-GYN's who deliver the babies.

Do my Australian readers downunder have any insights on this subject? That might give us some real answers whether holidays, heat or long nights is responsible for the birth patterns in the U.S and U.K?