More babies on Valentine’s Day and less babies on Halloween?!

(Source: Levy et al., 2011)

In the weekend edition of Le Monde, more precisely in the Sciences section, I read a report on a 2011 study made by Levy et al. who observed that the birthrate drops at Halloween and surges at Valentine’s Day… The above graph illustrates the fact for Halloween, with a significant [meaning?!] 5.3% decrease for spontaneous births. The increase for Valentine’s Day is 3.6% (still for spontaneous births). Even though those data are the result of a survey of all births in the United States over 11 years, thus unlikely to exhibit sampling biases,  I am fairly bemused both by the phenomenon and by the interpretation made in the study, namely that “pregnant women may be able to control the timing of spontaneous births” (while I find less astounding that “scheduled births are also influenced by the cultural representations of the two holidays“, even though there may be an administrative bias as well). Being unfamiliar with the U.S. procedure for delivery of birth certificates (and how much both Valentine’s Day and Halloween are of a public holiday), I wonder if this may be a reporting rather than biological bias…

Reading now into the paper (thanX, A.!), I see that “these holidays have the advantage of widespread participation, but without ordinarily resulting in the absence of physicians from work, as on certain federal holidays“, so my first idea that the Halloween gap [more pronounced than the Valentine surge] could be due to reduced medical or administrative staff does not seem so likely. The authors mention using an analysis of covariance model to build their significance test, adjusting for weekday and year effect, even though neither the model [what are the covariates?] nor the statistical analysis is provided in the paper. Looking at the equivalent of the above graph for Valentine shows more variability along the window of 15 days used by the authors. It would be fairly interesting to check throughout the years if other variations of that magnitude occur (and if they are always related to culturally significant days), before accepting the conclusion that “pregnant women can expedite or delay spontaneous births, within a limited time frame, in response to cultural representations“…

10 Responses to “More babies on Valentine’s Day and less babies on Halloween?!”

  1. Reblogged this on Learning From Data and commented:
    What did the witches do?

  2. […] column in Le Monde (Sciences) had most unjustly escaped my attention: it mentioned Thomas Bayes on the very front page and I […]

  3. Here is a graph of US births in 2002 by each day of the year:

    The data show a strong “holiday effect.” An analysis of the holiday effect, with reference to sceduled C-sections, is at

    • Thanks! What the Valentine study states is that there remains a significant effect on natural births after correcting for monthly and weekday effects.

      • Yes, it sounds bizzarre. The data that I have is not marked as “natural” and “other,” so I cannot reproduce their analysis on the US 2002 data. I wonder how many pairs of holidays they compared? Could this “result” arise from not adjusting for multiple comparisons?

      • Actually, can you tell me how the birth certificates are created in the US? In France, the father (or a relative) has to go to the local city-hall and declare the birth time, the sex of the newborn and his/her first and middle names. This is the only official document attesting for the birth of an individual and, when one makes a mistake in this reporting, it is close to impossible to make a change later. For instance, one of my aunts’ last name was misspelled (from Eude to Eudes) by a civil servant and my grandparents had to fight for more than ten years for the mistake to be corrected…

      • Almost all births happen in hospitals. Before the mother is discharged, she fills out a form that is then submitted to the government by the hospital. The hospital fills out birth info such as dates/times/gender. The mother fills out her name, baby’s name, and father’s name (if known). (If baby’s name is not known by the time of discharge, I’m not sure what happens.) After the paperwork is filed (6-8 weeks?), the mother receives the birth certificate i nthe mail.

        Side note: At the same time (but on a different form), you can apply for a social security number for the baby. The SSN is a 9-digit identifier that the government uses to identify individuals for purposes such as taxes and government services.

      • Thanks, Rick! This means there are less chances of people cheating on the birthdate if the paperwork is done by the hospital… Maybe the 250 extra/less births making for the “significant” difference every year are home births?!

  4. […] in time for the holiday, X pointed me to an article by Becca Levy, Pil Chung, and Martin Slade reporting that, during a recent […]

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