Here is an interesting tidbit via Alex Tabarrok. It's a study of low birth weight in babies from Maxim Massenkoff of the Naval Postgraduate School. Here's the most basic US data:
US birth weights peaked in 1985 at about 3,350 grams (7.4 pounds) and then declined over the next 20 years to about 3,290 grams (7.2 pounds) Why? Massenkoff theorized that it might be due to small particle pollutants in the air. Here's how that turned out in several cities with high pollution levels:
No dice. In cities with high levels of PM2.5 particulates there was virtually no difference in low birth weight babies. But maybe US cities aren't bad enough enough to show a strong effect. Here's the same chart for some of the world's most highly polluted cities:
Again, no dice. There's just no systematic difference between particulate air pollution and low birth weight. Massenkoff is perplexed:
In addition to the birth weight estimates in this paper, it is striking that Goldin and Margo find normal birth weights by today’s standards in a 19th century poor house. Are birth weights unusually hard to change? While far from exhaustive, I searched all reviews of randomized trials in the Cochrane Library targeting either birth weight or low birth weight. According to meta-analyses, many treatments come up short, including: zinc, calcium, deworming, vitamin E, vitamin A, vitamin C, iodine, and magnesium
On the other hand, the reviews find either increases in birth weight or decreases in low birth weight for: folic acid, vitamin D, omega-3, and anti-malarial bed nets. Also, birth outcomes within the US still vary substantially across groups: There is a stark income gradient, with mothers in the bottom income quintile more than twice as likely to have a low birth weight infant compared to mothers in the top quintile of income suggesting that access to resources could drive poor birth outcomes
So it's a mystery. The answer, of course may simply be that high levels of small-particle pollutants have lots of ill effects, but low birth weight just isn't one of them. But it's still odd.
Sorry if you've explained this before but -- how do you make your (excellent) charts, based on papers?
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Maybe birth weights are lower due to improved gestational diabetes treatment?
Are there enough diabetic people among those who are pregnant to move the average far enough to be significant? I doubt it.
Maybe. 6-8% of pregnancies involve gestational diabetes which does increase birthweight by quite a bit (maybe as much as 25%). This would increase the average weight by about 2%.
Maybe it has to do with the rise of highly processed foods and/or the production of low-quality, high-quantity raw foods.
Does birth weight correlate with gestation time before birth (or removal otherwise)? Are stillborns and abortions factored in? How about BMI of the individual (male or even female) that the baby came out of?
And how about lead levels in the environment?
Start out looking for a simple answer to a complex problem. Sometimes you will be frustrated by not finding the simple answer, and sometimes you will be fooled into thinking you have discovered the simple answer. More likely than not, you have been fooled by overlooking an unknown factor that has correlation to both the effect and the putative cause.
That's why the problem was complex in the first place!
The best chance to find an explanation would be to investigate correlations like income to birth weight, race to birth weight, employment status or job quality during pregnancy etc.
We know already that some of those are real.
Lowering of birth rate - post 1985 - could simply be due to changing demographics, where the United States has more people with a gracile frame than before.
Well, my birth weight was five pounds, five ounces. Full term.
Mom smoked.
Of course, there are probably umpteen variables to birth weight…
I wonder if the age of motherhood increasing results in lower birthrates. Take a look at this CDC report which shows that the "Average Age at First Birth up More Than 3 Years From 1970 to 2000".
https://www.cdc.gov/nchs/pressroom/02news/ameriwomen.htm
Some nice charts here: https://www.cdc.gov/nchs/data/nvsr/nvsr51/nvsr51_01.pdf
Why did birth weights peak then decline? C-sections. Increasingly common so you have a lot of weaker babies that should have died survive and they are going on to have weaker smaller babies. C section also happens earlier so low birth weight.
It's true that C-sections have increased dramatically (from ~20% thirty years ago to ~ 33% now) but I don't think baby mortality rates, or the number of stillbirths prevented are high enough (in either case) to really move the needle. However this is certaibly one place to run the numbers,
Two other places I would look are
- age of the mother
- comparison of first vs subsequent births. Maybe first babies are usually a little smaller, but many more mothers are now having only that first baby?