Different Kinds of Moms Have Babies at Different Times of Year

(Photo: Dave Herholz)

We’ve written in the past about the relationship between a child’s month of birth and a variety of later outcomes. In SuperFreakonomics, for instance, we wrote about research by Douglas Almond and Bhashkar Mazumder showing that “prenatal exposure to Ramadan results in lower birth weight.” In a Times column called “A Star Is Made,” we examined the link between birth month and accomplishment in sports. We also noted, however, in SuperFreak, that the sports advantage — and probably many other birth-month influences — are relatively small:

But as prevalent as birth effects are, it would be wrong to overemphasize their pull. Birth timing may push a marginal child over the edge, but other forces are far, far more powerful. If you want your child to play Major League Baseball, the most important thing you can do — infinitely more important than timing an August delivery date — is make sure the baby isn’t born with two X chromosomes. Now that you’ve got a son instead of a daughter, you should know about a single factor that makes him eight hundred times more likely to play in the majors than a random boy.

What could possibly have such a mighty influence?

Having a father who also played Major League Baseball. So if your son doesn’t make the majors, you have no one to blame but yourself: you should have practiced harder when you were a kid.

That said, there is a rather large body of literature on the topic of birth month and its relationship to later outcomes. Which is why it’s interesting to see a paper (working version here), just published in The Review of Economics and Statistics, which offers a different angle on all this birth-month conversation. The authors are Kasey S. Buckles and Daniel M. Hungerman:

Season of birth is associated with later outcomes; what drives this association remains unclear. We consider a new explanation: variation in maternal characteristics. We document large changes in maternal characteristics for births throughout the year; winter births are disproportionately realized by teenagers and the unmarried. Family background controls explain nearly half of season-of-birth’s relation to adult outcomes. Seasonality in maternal characteristics is driven by women trying to conceive; we find no seasonality among unwanted births. Prior seasonality-in-fertility research focuses on conditions at conception; here, expected conditions at birth drive variation in maternal characteristics, while conditions at conception are unimportant.

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> we find no seasonality among unwanted births

They actually mean "unplanned" births. Planned births are intended to be conceived at the time of conception. Wanted births mean that you wanted that many kids, even if you didn't exactly want them right now.

There does not appear to be an academic word for "given up hope of ever being able to have children, and unexpectedly got pregnant". I hear "incredible miracle" fairly often from the parents.


Teachers usually give birth a couple of month before the summer vacation...


Two words: Spring Break


This and today's WSJ article on eating in pregnancy point out something we docs already know: GIGO: That many studies are garbage because they don't take other variables into consideration.
The bad news is that under Obamacare, the "experts" are telling us how to practice medicine by using these studies: Often mixing several garbage studies to tell us what to do.
Hello! And what makes it worse is that they assume our patients actually take their medicines and obey their doctors.

caleb b

So does every hospital in America use the same supplier for baby blankets?!?!?

That multicolor foot blanket is the real story. Someone has a monopoly on those!

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I wonder if they separated out older mothers. A previous post said that the recession cut births among younger women but not women in their 40s, so wouldn't seasonality have a similar lack of effect on women in their 40s?


Actually there is such a thing as an unwanted birth and it's made more depressing by the fact that these women had no access to birth control or a doctor in order to prevent the unwanted pregnancy. If they can't prevent an unwanted pregnancy then the likelihood of a woman in such a dire condition of getting an abortion is worse.

To claim the pregnancy/birth is "unplanned" means that they did want to have a baby but implies they didn't know when and adding they weren't able or responsible enough to plan a pregnancy. Quite frankly it calls those women irresponsible because in order to have an optimal pregnancy with the best possible outcome you need to take supplements, exercise and eat a healthy diet months (preferably a year or two minimum) before conception.

If a woman is pregnant and doesn't want to be then yes, it's an unwanted pregnancy. If people are going to try and lie to themselves and claim all pregnancies are "wanted and just unplanned" then there are a lot of kids in state homes that prove otherwise Enter your name...