Babies are getting smaller

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Not the actual babies, but the birth weight of the new ones.

Just a snippet; I’ve not looked into this yet. But given that human size goes up and down over secular time, this probably would mean (if it means anything at all) that we’ve peaked out with whatever causes that cycle:

Birthweights in the US are falling but no one knows why, according to a study of 36.8 million infants born between 1990 and 2005.

A 52-gram drop in the weight of full-term singletons – from an average of 3.441 to 3.389 kilograms – has left Emily Oken’s team at Harvard Medical School scratching their heads.


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33 thoughts on “Babies are getting smaller

  1. But given that human size goes up and down over secular time…

    But how does birth weight do against parochial time?

    Is this a conspiracy? 🙂

  2. Was the average health of the mothers static across this time?
    If the health of the mothers dropped, then that cold explain the drop in birthweights.
    Also, if a significantly larger proportion of pre-term births survived that could skew the statistics.
    And finally, 3.3kg is still within the acceptable range.

  3. Ales, there was no obvious drop in any known indicator. They factored out preterms and caesareans, and yes, this is not a medically significant effect, it is just a bit notable because it would be a reversal of a trend (if it persists).

  4. You would have to ask whether a 1.5% change is significant…and if so, how about:

    1/ measurements are getting more precise and health-workers’ training more rigorous, resulting in less chance of inflation – parents like to boast about their babies’ weight so midwives and nurses were likely to round-up a bit more than necessary. This rounding-up would be clearly visible by graphing the distribution of reported birthweights and comparing, say, 1975 with 2005.

    2/ the proportion of babies born to the wealthy and the educated is diminishing – the poor tend to produce lower birth-weight babies due to their higher intake of tobacco and other drugs during pregnancy.

  5. Note this drop in birth weight is correlated with the rise in in atheism. Obviously, the missing 52 grams is on the menu at atheist barbecues.

  6. Vince, it’s not 1. There has not been a change in how birth weights are measured or recorded in decades.

    It could be 2 … a shift (not necessarily SES but some kind of shift) between demographics. I could even be that the last big immigrant group arrived large instead of getting large, or the prior generation immigrant group got large faster than previous groups. This is a small enough of a number..

    I guess I have to read the damn paper now…

  7. Wow – a full 1.5%! The APOCALYPSE is HERE!

    Gee, that couldn’t have anything to do with the much higher survival rate of premature babies and the tendency toward birth via cesarian section (which is necessarily a premature birth). Naah, I like the end-of-the-world explanation better.

  8. Greater prenatal stress.

    In the first trimester that lowers NO levels and reduces mitochondria biogenesis in the liver and the placenta. Many growth factors have effects mediated through NO, so low NO reduces their effects too.

    Smaller placentas lead to smaller babies. Similarly insufficient mitochondria in the liver lead to smaller baabies too. Low NO also epigenetically programs for obesity.

    There may also be a better appreciation of gestational diabetes and the pathologically larger babies that can produce. Inducing a little bit early if the baby is really big might be enough to skew the average size.

  9. How about better home pregnancy tests giving more accurate dates of conception? My wife and I got a positive result much earlier than would have been possible 15 years ago. That might throw off the results. I’m too tired to actually look at the study and see if they considered this. It’s this damn baby.

  10. How about better home pregnancy tests giving more accurate dates of conception?

    1) That would not effect the weight of the baby unless the baby was holding the test kit; 2) they don’t use those data. Many people, and this has been true for years, confidently and probably accurately know the day of conception, but that is not routinely entered into the medical record.

  11. Now you are in MY area of expertise as a NICU RN! We have been weighing the babies the same way for years and years and years. While we may round up the lbs/oz for parents as well as length, the actual measurement (unaffected by parental pride) is recorded in the chart in grams and centimeters. We have been noticing that certain ethnic groups tend to have much smaller babies, unrelated to the age of the mother, educational level or social status of the mother, or health of the mother. But we do not know why either. If there is any question as to the age of the infant, we use certain developmental markers to determine the age. This has nothing to do with the size of the infant. Gestational diabetes control doesn’t have anything to do with the smaller size. GDM is a BIGGER problem than it was 10 years ago. We are seeing it in younger women than ever. And the women are not necessarily overweight. We do treat aggressively, but that is not skewing the weights. Pre-eclampsia and chronic hypertension cause lower birth weights. Smoking rates have been going down in the USA over the same time period. And yes, the 1.5% IS a big deal when you are starting out with a small infant! It makes a big difference in how we treat them.

  12. Many people, and this has been true for years, confidently and probably accurately know the day of conception

    Of course it’s true that some people have always accurately known the date of conception, but the point is that more people probably know more accurately now.

    but that is not routinely entered into the medical record.

    My wife had very thorough records of everything that happened around the time of conception with our first child. Taking that into account, our OB ignored the ultrasound estimates and always referenced our date for the remainder of the pregnancy. I can’t say how official our date actually became or how common this is, but it was definitely in our medical record.

  13. @Greg: I haven’t seen the actual report and the blurb linked to provides only a claim that cesarian section did not contribute. I’d like to see how the authors back up that claim; I’ve seen numerous unsubstantiated claims in biochemistry and medical journals over the past 40 years (and I have no doubt there were equally fantastic claims before that), so until I see the actual methodology my reaction is “oh sure, pull the other one”.

    Aside from lower weight due to earlier delivery, if cesarians are not counted but cesarian rates increase largely due to large babies being seen as greater risks, then the big babies are being removed from the population and that naturally is expected to reduce the mean weight of the population. Such a mistake in peoples’ calculations are quite common; most people (many scientists included) don’t seem to have a good idea what data should be ruled out and why.

  14. OK Greg. Here’s a way that today’s pregnancy tests could be an explanation for perceived lower birthweights that doesn’t involve self reported conception dates.

    The closer to the date of conception an ultrasound is taken, the more accurate it will be at pinpointing conception. The people that actually make this call know this, and give more cautious estimates the further along a pregnancy appears to be. This would mean that later ultrasounds skew towards saying a fetus is actually younger than it is. With cheaper and improved pregnancy tests, more women are finding out they’re pregnant earlier, and therefore getting earlier ultrasounds. This makes lower birthweights a meaningless blip caused by unintentionally comparing the birthweights of today’s babies with those slightly older babies of the past.

  15. Hmm. Never been a dad. But the notion of looking for unaccounted for variables is intriguing as hell.

    What about the length of time said baby spends in the cooker? I wonder just how much variability there is in term length, and how this might correlate to birth weight? How much weight does a baby put on as a function of time in utero? Surely this isn’t linear?

  16. Bill: OK, taking out the big babies that would have been, I like that.

    Jose, yeah, but the actual or at least different dates you would get than the arcane way they figure conception dates still do not enter the medical record.

    It is possible that fetal development and thus gestation length is being measured from ultrasounds.

  17. I haven’t seen the report and won’t until I get to the library (if my library has it), but is the distribution of weights the same?

    The problem may actually be worse, if there are multiple adverse things going on with some making babies bigger and some making them smaller.

    If you have more giant babies from gestational diabetes and more small babies from placental insufficiency, the average may be an inapropriate measure.

  18. re: #17 and #20

    That’s easy enough to test. Examine the medical records of all the ceasarian sections for full term infants, and find out what the indication for the procedure was, and what the average birth weights were, and see if there is skewing towards heavier babies.

  19. You can’t get the paper at the library (It is only in pre-release form and you have to be very very special to get a copy so far).

    Therefore it will not be blogged here until it is more available.

    I can tell you this: THe effect is stronger in non-at-risk groups, suggesting that my original interpretation is correct.

  20. It is possible that fetal development and thus gestation length is being measured from ultrasounds.

    That is what they do. I know it’s not used in all cases, but I’m pretty sure using ultrasound information in determining a due date is nothing out of the ordinary, especially in cases where a woman has an irregular period or doesn’t know the date of here last period.

  21. why isn’t the DNR doing something about these baby poachers? people shouldn’t be keeping all these underweight babies. the babies should be released back into the wild until they big enough!

  22. #24 HELLO!! Gestational age is not determined solely by ultrasound, and after a baby is born, you can usually narrow the gestational age down by examination of the development of the neonate. That is a part of a newborn assessment. The point is ….they are being born SMALLER. We are also getting more late term preemies with gestational ages of 36-37 weeks which may still be a pretty good size, but still need to be treated a little differently that full term infants.

  23. Greg, I have a comment in moderation, there were two links, one to the paper and another to a similar paper showing an increase in average birth weight over the same time frame in Australia.

  24. #24 HELLO!! Gestational age is not determined solely by ultrasound

    No shit. I clearly didn’t claim it was determined solely by ultrasound.

    The point is ….they are being born SMALLER.

    No shit. But the point of the study isn’t what you seem to think it is.

    We are also getting more late term preemies with gestational ages of 36-37 weeks which may still be a pretty good size, but still need to be treated a little differently that full term infants.

    The study is not looking at a decrease overall birthweight. Unfortunately all I have available is the abstract, but if I understand it correctly (and I may not), it appears that they attempted to control for everything they think might cause a difference in birthweight, and inexplicably still found a difference. So the cause must be something that was not obvious to them. I’m positing that they may have erred by unknowingly comparing babies of different gestational ages, and that might account for the difference. What’s your genius idea?

  25. Jose, I don’t think a comparison of birthweight (the weight at birth) can be affected by gestational age becaue they are comparing the age at birth. Taking out the babies that 20 years ago were born big because the pregnancies were not sped up to avoid problems (now routine in US hospitals but not done to the same extent even 10 years ago) could explain this.

    But it is also possible that this is just a small random “down” that will be equivilant to small random “ups” that will happen over the next several years as we have peaked out in birth weight and are now in a secular holding pattern.

    Untill the Teabaggers and Republicans and Libertarians take over and the US finally becomes a third rate “developing” nation with no health care and the babies start getting born more smaller, more sicker, and more deader, and more hopeless.

    Not to inject politics into it or anything.

  26. Maybe the out-breeding has stopped having its effects. After two centuries of Swedes marrying Germans, or English marrying Italians, the melting pot has , um, melted, and the the population is returning to the norm? Plus, of course better survival rates of sub-normal babies?

  27. Well, I like the idea of returning to the norm though I’m not sure what the reason is. I still like both the idea suggested above of fewer extra large babies and the idea I suggested of the immigrant stream including pre-growd folk and/or people who are just plain of short stature and thus don’t experience the usual transition.

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