Shrinking babies

Harvard researchers are reporting in the journal Obstetrics & Gynecology that the average birth weight in the U.S. has actually dropped over the past 15 years. The study looked at more than 36 million full-term births between 1990 and 2005. After controlling for confounding variables, it was found that birth weight had decreased by 52 grams. This trend ran counter to that found previously, which was that birth weights had been steadily increasing over the last century. A secondary analysis suggested that the drop was not due to a change in maternal demographics, as the trend was also found in a subset of white, well-educated non-smoking women as well.

Most people assume that people just continuously get bigger over time in stature (and waistlines), and this should include birth weight. Visiting a colonial era New England house, with its short ceilings and door frames, one gets the sense that people used to be smaller. Data from the U.S. and various European countries reveal how much height has increased over the past century. However, the physical growth in height, weight, etc. of a population generally follows its economic and social conditions (what James Tanner called “the material and moral conditions” of a society). When times are harsh, average size declines.

I’m hesitant to speculate what the ultimate reason is for the decline in birth weight, but it may be that something significantly detrimental occurred at a societal level between 1990 and 2005. Pinning down exactly what that is may be difficult, but it is an indication that life in the U.S. doesn’t inexorably improve, and this impacts our biology as well as our bank accounts. To me, what seems most concerning in the Harvard study was the slight uptick in the rate of low birth weight babies. There is a ton of research indicating that low birth weight increases one’s risk for developing many chronic diseases later in life (cardiovascular disease, diabetes, osteoporosis, etc.). The repercussions of this drop in fetal and infant growth may be felt for decades.

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