Do you think individual women’s fertility varies by time of the year?

It seems like siblings who share a birth month are more common than would be expected by chance (wrote the mother of two April babies.)

People have mused about this subject here on chatboards here and here.

On the other hand, since there are only 12 months in the year, this should happen fairly frequently.

At the population level, it is well-established that seasonal variations in birth rates exist, whether in the United States or Europe, Mali, Bolivia, or New Zealand.

Overall, in the United States, birth rates peak for northern states during the spring and summer and in the autumn for southern states according to one recent study, unless you are Amish. (I doubt any of our readers are Amish, but perhaps you enjoy reading about them, as I do.)

Meanwhile, in Europe, births now peak during the last three months of the year, and the reasons of this were perplexing to the authors of one recent study.

Many theories have been put forth to explain birth seasonality, related to differences in air pollution, nutrition, levels of viruses like influenza, and air temperature, to list a few. None has been clearly proved, from what I could glean.

And none of this explains whether individual women are more fertile at particular times of the year, or whether siblings are born in the same month more often than would be expected by chance. Currie and Schwandt published an article last year showing the individual women’s characteristics do have a strong association with timing of pregnancy. In a finding that strikes me as completely counterintuitive, birth timing varied notably by women’s race, educational level and socioeconomic status.

Perhaps this is the takehome message: you can chart your temperature and your cervical mucus all you want, but remember that there are poorly understood environmental factors at play, and they also seem to influence the outcome. 

Also, none of this explains this widely observed phenomenon: if you’re trying to conceive, and you go shopping at Target, if will seem like every third woman there is pregnant. Just wanted to warn you about that.

If you want summer to be your season of pregnancy, but apparently it’s not, you may feel disappointed. First, try to remember not being pregnant during the summer has some upsides.

Let’s list a few here:

  1. Not having to squeeze into a bathing suit. Buying a maternity bathing suit feels like a waste of money. Even if you are early your pregnancy, it’s pretty hard to hide a pregnancy in a bathing suit. In general, it’s harder to hide a pregnancy in summer clothes, and this can be a concern if you are trying to keep a pregnancy to yourself during the first few months.
  2. Not having to be a pregnant wedding guest or bridesmaid. Again, frustrating to buy a dress that will get little wear. Hard to avoid nosey questions about your fertility. (If you are really concerned about how to handle social situations while pregnant, we have a post addressing that here.)
  3. Frosty cold alcoholic beverages. A nice part of summer that pregnant ladies must avoid.
  4. Being able to use some vacation time. As you probably already know, American women are not guaranteed paid maternity leave. Those of us who are at least lucky enough to have paid vacation and sick time generally have to hoard that time during pregnancy in order to apply it to maternity leave to avoid having a massive income reduction during maternity leave. 
  5. Not experiencing how oppressive heat and humidity are during the third trimester and how it makes it even harder to sleep. That’s what we have been told.

Finally, especially if you live in the northern United States, remember that your chances of getting pregnant during the summer are better than average! Elevate those hips! Or don’t.

On the other hand, some our readers are pregnant now – and apparently especially those of you living in the South. Congratulations to you! Enjoy eating ice cream and feeling lighter while swimming. (Hopefully you were able to find that suit on sale.) Look forward to either autumn baby strolling, which is lovely, or being really pregnant during the colder months, when that fetus will keep you nice and toasty.

And Happy 4th of July to our American readers!

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