How Common Is Your Birthday?

UPDATE: I’ve written a clarification about this post here. Please read it

A friend posted an interesting data table on my Facebook wall yesterday, which was my birthday. The data listed each day of the year with a ranking for how many babies were born in the United States on each date from 1973 to 1999. Some interesting trends are evident in the data. Apparently, people like to make babies around the winter holiday season because a large proportion of babies are born in September (ours is due Sept. 24, btw).

Sept. 16 was most common. Feb. 29* was least common. This heatmap is an effort to visualize the trends, with darker shades representing more births:

Data source: NYTimes.com, Amitabh Chandra, Harvard University

Follow Matt @Stiles on Twitter.

* A previous version of this post incorrectly listed Jan. 1 as the least common birth day. 

188 thoughts on “How Common Is Your Birthday?

  1. Nice example, though I’m going to give a little curmudgeonly critique in hopes that it will improve the presentation.

    1. The legend does not match color elements in the plot. The plot goes from light green to a fully saturated black, while the legend is only on a grey scale.

    2. It seems common for these types of heat maps to not have numeric categories listed in the legend, but this is always a poor decision. What is more common and what is less common? One could effectively promote any propaganda one wanted to without associating numbers to colors. In this case it is ranks, but lets say you were using an estimate of the actual numbers of births, you could make any trivial difference between “more common” and “less common” as dramatic as you wanted to.

    3. This is minor, but people typically associate reading graphs from left to right as [further left = lower] and [further right = higher]. Thus your legend is backwards from typical gestalt way people read graphs.

    • Thank you for your points, Andy.

      Re: 1 & 3., I tinkered with the legend a bit this morning. On my screen, the darkest shades look black, so I didn’t realize the dark end of the gradient didn’t match. It’s still not perfect, but neither are my Illustrator skills. I hear you re: point 2, but this case does in fact involve ranks. But I could have been more precise, I suppose.

      Anyway, thanks again. Hope to see you back here as I continue experimenting. Nitpicking keeps me honest and helps me learn.

      • Wow you are very fast! Thanks for the update, and I enjoy your posts so keep them coming.

        As far as the continuous legend, if you are generating the original graphic in R using the heatmap function I give some references in this answer on the cross validated Q/A site in regards to generating a legend with the heat map, http://stats.stackexchange.com/a/17930/1036.

        I can’t give any advice on illustrator though.

      • Wow Andy – you’re kind of a tool …. I think it looks just fine.

      • This may be just for the US. We run a birthing pool company and we are busiest around April, June, September and December. A lot of it corresponds with UK school holidays and the September babies are jokingly blamed on office Christmas parties.

  2. Great visualisation. Is this data northern hemisphere only? Presumably southern hemisphere data would be the opposite ‘polarity’

      • This is purely anecdotal, but there seem to be a huge number of birthdays around March/April/May in Australia (I’m one of them); as someone further down noted, that corresponds to a lot of conceptions occuring in our winter.

  3. Notice that, much like dying, expectant mothers seem to be able to hold it in during Christmas, and then childbirths spike almost immediately after.

    • I wouldn’t say that they hold it in over Christmas, but they force them out before New Years. Gotta get the tax deduction on the books.

        • I didn’t hold it in or pop out out before new years. My first was born on Christmas Day. My seconds was born on New Years Day 2000. Neither were inductions or c-sections. Both have very unique birthdays.

    • Check out my birthday–July 4th. For two days, women cross their legs and enjoy the fireworks.

  4. Wouldn’t Febraruary 29th be the least common birthday since it occurs only once every four years?

  5. I wonder how the fact that for planned caesarians are becoming more common affects the latter end of this scale. I find it incredible that July 4 and 5 have so few babies born when the dates around them have so much more.

    • Clearly there’s some of that at work. Remember, though, that these data are from 1973-1999, so the relatively recent trend toward planned caesarians and timed births might not be as much of a factor. But it’s probably a factor nonetheless. This dataset is just so fascinating to me, not just because our baby is coming soon — in September.

      • With a reasonably high proportion of births induced, obstetricians would never schedule an induced birthday to mess with their 4 July holiday! But it looks like Labour Day is…labour day..

      • Let me start by saying very cool! I used to think my date was rare (late Sept.), but it’s kind of weird to see it is about the most popular time!

        But I disagree with Cesarean sections being insignificant. They have been steadily increasing for 100 years. I couldn’t find the exact numbers per-year, but in the UK the number of Cesareans was 9% in 1980, but 25% in 2010. That equals about a 0.54% raise per year.

        1965 was the first year that statistics for C-sections were compiled for the US. In 1965, the % was 4.5%. In 1996 it was 20%, and almost 33% in 2011. So, for some reason they are more popular than in the UK.

        This chart is also not capable of adjusting for induced labor, a practice which has been common for decades: about 10% of all births in the US were induced in 1990, and 22% in 2006. How much overlap with C-sections there is I don’t know.

        Let’s make some assumptions in order to make an example of how large this issue is. If we set the induced % to 14% in 1996, and assume half the inducements ended in C-sections. Let us also assume that 50% of C-sections were non-induced emergency C-sections. This means that, in this example, 17% of all births in 1996 were on non-natural dates (this is about 663,000 people). While it is only an artificial 1 year of your sample set, this would have a non-trivial impact on the results when adjustments are made for each year in the data set.

        So, while I agree with the general trends, there is in fact a very large number of people who are not born on what would have been their “natural” birthday, and this chart would look generally similar but quite different if it was non-induced vaginal births only.

    • As a proud July 4 baby, grandson of a July 4 baby, I was impressed by the odd low pressure area on that day. I guess most women hold it in?

        • From the time my Mom discovered she was pregnant, she just knew that I would be born on the 4th of July, even though the Dr.’s calculations gave a different date.

          Guess who was right?

    • The issue is probably planned induction (which is and has been quite common), not planned c-sections (which is more rare). Inductions have been common for a lot of reasons both medical (pre-eclampsia) and nonmedical (wanting a certain OB available for the birth). In fact, “elective inductions” (for nonmedical reasons) have been quite common and are now discouraged as they may contribute to higher risk of an unplanned C-section.

      That the “July 4″ gap also extends into “July 5″ supports that it is scheduled inductions and not scheduled c-sections (since July 4th inductions also result in July 5th babies).

      C-section rates are certainly seriously high, but most of these are an “unplanned” complication of labor (e.g. labor failing to progress — not an “emergency” but also not scheduled). Scheduled C-sections are rarer and mostly occur for rare issues like placenta previa and breech.

      • I know PLENTY of planned c-sections, more planned than unplanned. Seems to be more for medicaid moms and teen moms. Many that I’ve known are told around month six when they will get a csection (or less common, induced). And it is usually before even week 40.

  6. For years I have thought that astrology had it wrong. I felt that the moment of conception was more important than the time of birth. This chart places a great deal of conceptions in Oct, Nov and Dec. There is a saying,

    “When it is hot and sticky it’s no time to dip the wicky
    When the frost is on the pumpkin, then it’s time for wicky dunkin’”

    Folklore and old wives’ tales have some merit after all.

  7. I was born on July 4, two weeks before my due date. My mother told me that her OB was kind of pissed to have his vacation interrupted. It’s not just planned c-sections, but back in 1970, there was a similar trend in induced labor, though less about hitting/missing certain dates and more about doctors wanting to clear their schedules. It’s the same reason a coworker who was due on Christmas had her daughter on December 23 – because her doctor was going to be out of town for the holidays. It’s also the same reason the first week of September is a little light – Labor Day Weekend is doctors’ last vacation period for almost three months. I’m sure there’s a joke in there somewhere.

  8. A couple of questions arise for me:

    – it looks like there’s quite a bit of control over the dates here – it’s very easy to pick out the absence of births on the 13th of the month, and holidays like Thanksgiving, Christmas, and the 4th of July. Also the romantic births on Feb 14. Is this all due to induction/c-section scheduling?

    – there seems to be a diagonal pattern throughout the whole dataset that corresponds to weekends, or at least the cycles of the 7-day weeks, rolling through the months. This seems odd though, since it’s a dataset of so many years… shouldn’t that average out?

    • I had the same observation. If this were for just ONE year’s births, and babies could be counted on to arrive relatively close to on-time/expected, those 7-day spikes could be postulated to reflect increased conception on weekends. But spread over the number of years that this data purports to be? I’m not sure that correlation holds. Curious, though, isn’t it?

  9. So, how big is this effect? How much difference between say the top 10% birthdays and the bottom 10%?

  10. Something odd about this map which hints at some element of reporting bias – can February 14 really be that much more common than any other day in the first six months of the year?

    • It’s 9 months from Mother’s Day (In the U.S.). I heard recently that couples with children have more sex on Mother’s Day, which makes sense. Dad pampers Mom and treats her like a queen for the day…and well, you can figure out what happens after the kids go to bed that night :P.

      I’m the second child and I was born on February 14th. I’d be interested to see if babies born on Valentine’s Day are usually the second born or later. I suspect that they are, since people who have sex on Mother’s Day as a special occasion obviously would be parents already. Kind of gross when you think about it in terms of your own parents though!

      • Sorry, “9 months FROM Mother’s Day” sounds stupid. I meant that if you were born on Valentine’s Day, you were probably conceived in May (Mother’s Day being the major holiday) since the next Valentine’s Day would be about 9 months later.

  11. The weird blankness on July 4 and 5 and in Christmas week shows that this is influenced by other factors. Next step: find data from other countries with different holiday patterns.

  12. Hi. I like this but I have one issue. Feb 29 is bound to be the least common birthday, as it occurs only every 4 years. So it’s an “artefact” of the calendar, not a meaningful statistic. It makes no sense to report it as the least common birthday.

    Either Feb 29 should be excluded from the report, or its births added into Feb 28 and then the Feb 28 total adjusted down by one-fifth.

  13. I created an interactive version of Matt’s heat map with Tableau.

    http://vizwiz.blogspot.com/2012/05/how-common-is-your-birthday-find-out.html

    I went through the comments and did a bit of digging.

    For the last comment on the top and bottom 10, all of the top 10 days are in September.

    https://picasaweb.google.com/lh/photo/n5i_iP-OXIomrolxuvZa5NsVbml2PFATlhORTCWirQU?feat=directlink

    The bottom 10 are all around major holidays (Jan 1-3, Jul 4, Nov 26-28, Dec 24-26). Feb 29 was in the list too, but I discarded that since it only occurs every four years.

    https://picasaweb.google.com/lh/photo/3oGL6-PHvPsA62GDhGArHNsVbml2PFATlhORTCWirQU?feat=directlink

    I think all of the comments posted prove that this was an effective heat map by Matt. It’s tough to design a viz that allows for immediate insights, and Matt has done that.

  14. Tableau as a tool or how I used it to make this heat map?

    Also, as an FYI, when I click on the Reply button in the emails I get for comments, I get an error.

    • No, just the heat map. I didn’t know you could built one with Tableau.

      Please forward me a copy of the email so I can work on the bug. Email is mattstiles [at] gmail dot com.

  15. Thank you! As an astrologer, I have always wondered which signs were preponderant among us
    Clearly there are more Cancers, Leos and Virgos than any other signs. Mind you, this chart is only for the US. As someone pointed out, Southern Hemisphere would likely be the opposite. If you can do a heat map of ,say, 60 years of births, I would be delighted to play around with how many Rats there are as opposed to Dragons and Monkeys and Pigs in the world. Cheers, Suzanne White

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  17. Nice graphic, but are the differences large or small? I rather doubt that “a large proportion of babies are born in September.” I’d guess that the proportion is pretty close to 1/12. Using ranks throws away most of the information in the data, with the result that Feb. 30 looks similar to Feb. 29 which is barely distinguishable from Jan 31, but highly dissimilar to Sept. 30. I’d be willing to bet that the difference between Jan 31 and Sept 30 is tiny, while Jan 31 is about 4x Feb 29 and Feb 30 better be zero!

  18. Did anyone notice the striking decrease of births on the 13th of every month?

    Lots of superstition going on here…

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  24. Ironic that the lowest day for having babies (12/25) is also the the most popular day to make them, based on the huge spike mid-late September.

    I’ve also read that intercourse can help induce labor. Might explain all the 2/14 births. Late term couples give it a go Valentines Day, and are having a kid that night.

    What I find odd is the spike before and after Christmas. Just people holding it in,or is Easter a popular time for action?

    • It would be fascinating to compare the least-common dates in modern times (December 25th, for example), with data from before scheduled C-sections and inductions were so common. I bet you’d see far less of a “holding it in over the holidays” effect?

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  27. Lol your baby is due on my birthday September 24! Which I found out is pretty popular.

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  30. Is this just the US? Just North America? I would expect these trends to be different for other places, esp. on the other side of the globe. Otherwise… weird.

  31. What this heat map could do next is track back roughly 9 months from the hottest dates and the least hot dates to determine the optimal times each year that consenting adults are procreating.

    Why did more adult have intercourse 9 months prior to the hottest dates? Were external factors such as holidays and weekends a contributing factor?

    Why did people have little to no sex 9 months prior to those dates with less birthdays on the heat map?

    NOW that would be fascinating.

    My un-scientific guess is that most of the births that occur from mid to late September to early October are the product of some post-chocolate and roses- Valentine’s Day nookie. You be the judge, just a trend I’ve noticed over the years with increased birthdays occurring at the time of the year.

    You also see the same spike in births in July and August- could be Christmas hookups at the holidays party or perhaps that kiss at midnight lead to more intimate New Year’s resolutions.

    The lack of “heat” in late December thought February suggests that we all knock boots a little less in May, June and July. Too much work and not enough holidays and reasons? Spring fever? Warm weather?

    Come on this is the fascinating part of birthdays— not when you were born, when were you conceived and what factors contributed to that special moment.

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  33. This is super-interesting… but it bears mention that it’s hard to interpret without actual numbers of births on any given day. For all we know, the difference in the numbers of babies born on the most-popular and least-popular days is exactly 366 total babies across all 27 years — in other words, the differences might be relevant for a ranking table but essentially irrelevant from a statistical standpoint.

    Is the original data available anywhere? I’d *love* to see a heatmap of that data — I suspect that we’d see a lessening of the impact of some of the major differences in your heatmap, as well as a few other trends or differences that pop out of that data.

    • Thanks for your comment. I agree. I just got the data by month back to 1970. I’ll post something soon. Still working on getting the day-by-day birth totals. The Harvard professor who has the data is out of the country and can’t access his data archive until the fall.

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  38. I think the spike in late December is from women wanting a last peaceful Christmas AND a US tax deduction.

  39. Interesting how February 14 stands out in an otherwise less common month. I’m wondering if couples with due dates +/- say 3 days of Feb 14 choose to have their baby delivered on Feb 14 to have a Valentine’s Day baby. Same goes for with how significantly less births there are in the days leading up to Christmas compared with the week after.

    I want to tread lightly here, but I wonder what this says about elective birth around significant holidays.

  40. Thanks for the great vis. If you update this, two things you can do to smooth out the weekly trend:

    * add in data from 1972: the data here are from 27 consecutive years, whereas you want 28 to ensure that most days of the year fall the same # of times on each day of the week. (if you add 1972: true for days on/after March 1).
    * give the same weight to each year’s data. That is, find the % of births in each year that happen on a given day; then average that over the 28 years. Otherwise the latest few years, with more total births, will have more weight than the previous ones.

    • Not that I have seen, but now that you mention it I will seek out the data. I’ve heard anecdotally that deaths spike after holidays and the new year. Not sure if it’s true.

  41. I’m late to the baby party but congratulations! My wife is due around the same time as yours (incidentally?).

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  48. I wonder if a high percentage of those September babies are Chinese. The number nine is considered very lucky in Chinese culture & many families try to schedule births in the ninth month. Expect a spike of superstitious births this September, in this year of the dragon.

    • Haha. We’re having a dragon baby this September, but something tells me you already knew that.

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  52. This is only for a fairly small period of time. Almost every year, it seems I read on October 5, that it’s the most popular birthday. That’s because they must be taking into account all Americans who are alive. If you take into consideration that about 105 is the oldest most Americans seem to live (about) and that 1973-1999 is a 26 year span, you are only using 1/4 of the available years to plot (albeit with fewer people alive to have birthdays the closer you get to105 years back). Technology advances and medical practices surely influence births in more recent years. It would be interesting to see this map for all years.

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  54. Interesting work, thank you
    Is there enough data to find trends like September becoming less “popular” in recent decades/centuries?

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  57. A comment from the Southern Hemisphere, sunny South Africa – I don’t have stats to go but do deal with numerous birthdays annualy (friends, family, customers) and would say that your chart are very much in line with what I would produce if I do have to draw a chart for every person I deal with. (e.g. 3 of my customers are going on maternity leave in August for September and 1 came back in May).
    I always thought that it is due to the major holiday being in December (around the 5th to the 10-15th of Jan) and year end functions and so forth that we have more birthdays in September (9 months later – espcially between 15th and 25th).

  58. A good use of heat map in explaining least discussed topics. Good work!!

  59. Wow! so interesting! Which geographies is this covering? It seems that the 13th stands out as the least likely date of birth!

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  65. so this means more sex happens in November than any other month? i wonder why the ladies want it sooo baaad in November?

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  68. I’m also from Southern hemisphere (Australia) and agree that much of the pattern anecdotally seems to hold here, particularly September births. I’d be interested though to see if there is a difference in the late December stats, as our financial year for tax breaks etc. runs from July to June. I gave birth on 27 December last year in a near empty maternity ward, but did notice things getting busier as New Years got closer (or was it Christmas being further away?) Really interesting stats and map though!

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  70. I am amazed how Christmas actually appears to delay childbirth but up to three days. But I’m more confused by the light patch running through the 13th of each month. Do you really think mothers hold it in due to fear of the number 13?

  71. So, this would seem to imply that most conceptions are occurring during October, November, and December.

    With a bias to the Northern Hemisphere, it would seem to suggest that winter is another season, another reason for making whoopee…

  72. This is really clever and shows off some trends. However, my biggest issue with this is that the source data is ordinal, not linear values. Heatmaps are most effective when they’re displaying data based on linear values.

    This means that there may be a huge difference between the 10th most frequent day and the 11th, but you wouldn’t know if because their values are their ranking. There could be no difference or only slight differences for 100 days, and again we wouldn’t know because the real separation between data points has been masked by their ranked order.

    For example, let’s talk about the ages of people in my family. My wife and I are 36, (I’m 2.5 months older) then I have a 5 year old and an infant. That means that there’s the same ordinal difference between myself and my wife (2.5 months) as there is between her and my oldest child (31 years).

    • I agree. I’m working on getting the raw data for a better visualization. This is something I did on a Saturday afternoon for fun. I wasn’t seeking to publish a scientific paper, and I had no idea it would explode like this. Stay tuned…

      • OK, then a fun project it is! :)
        Can’t wait to see the revised version.

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  81. I think, A circular graph would be more natural. or at least a straight line. To join the end of a month with the start of the next month. I prefer bar graph instead of this kind of graph, because with a bar graph, is possible to make more evident (pop-ups quickly from the screen to the viewer), July 4 and 5 is suspiciously low in frequency respect his neighbour (in this graph they are mixed with June 2,3,4,5 and August 2,3,4,5 wich is not so important to focus on)

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  86. In the article the author mentions that “Feb. 29* was least common.”

    Perhaps the author should’ve noted that Feb. 29 only occurs in LEAP YEARS, that may have something to do with low birth rate on that day, no? Of course it does and if this were a student of mine, I would’ve dinged his paper for leaving that tidbit of information out of the paper especially since I was born on Feb 29, no other reason really, just me being a green meanie!

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  89. This means that peopla has more sex on November, December and January??

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  92. My birthday is February 14th and I’m always shocked by the number of people I know that share it. Others on here have also questioned why it’s so common, so I’d like to throw out my idea.
    It’s 9 months from Mother’s Day (in the U.S.). I’ve heard that couples with kids are more likely to have sex on Mother’s Day because Dad pampers Mom for the day, etc. I also wonder if more parents actually TRY to conceive the night of Mother’s Day….maybe they’re feeling loved up after the special day with their kids and are inspired to make more?
    I’m the second born out of my siblings and I suspect Valentine’s Day babies are usually later in the birth order (not first born), since people who have sex as a special occasion/try to conceive on Mother’s Day would likely be parents already. Or at least I hope they would be…
    Not exactly revolutionary thinking since holidays seem to show a rise in conception, but still interesting to consider. Although it’s certainly unpleasant when you have to consider it in the context of your own parents!

    • Sorry, just realized saying “9 months from Mother’s Day” is silly/unclear. What I meant is that if you were born on Valentine’s Day, you were probably conceived around mid-May since that would be 9 months prior.
      I also realize it’s less likely that everyone conceived on Mother’s Day would be born exactly on Valentine’s Day rather than during that general time frame. Perhaps the logic can be extended to say that there’s a rise in conception not just on but also soon after Mother’s Day due to the sentiments surrounding the holiday. There also could be other factors at play as to why Valentine’s Day in particular shows a dramatic rise as compared to that entire week. Perhaps there are psychosomatic factors influencing the mother’s body to go into labor on Valentine’s Day if she’s due around that date (or as someone else theorized, women who are having induced labor choose that date).

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  105. Where was the sample data pulled from? The USA? I was wondering if geographic location (hence weather) has anything to do with timing. In Canada we seem to have even more births in those summer months. A workmate born in the tropics say he did not see any such correlation there. Thoughts?

    Kirk

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