Birth weight is often used as a rough gauge for the quality of the prenatal environment. A newborn who weighs 2500g or less (about 5.5 pounds) is considered to be “low birth weight” (LBW). At the individual level, weight alone is an imperfect measure because of confounders such as gestational length (it’s axiomatic that the less time spent in the womb, the less time there is to grow). However, at the population level, if average birth weight fluctuates, then it is an indication that something in the environment probably has changed.

Sometimes, stressful changes can be low-intensity and chronic; at other times, they can be abrupt and dramatic. Biologists, psychologists, and bioanthropologists might call these changes “stressors” or “insults.” Economists might use the term “shocks.” They’re both getting at the same idea: to what extent can harmful environmental factors affect growth and health outcomes? 

In the case of a natural disaster, the harm done can be substantial. Florencia Torche (2011) found that rates of LBW increased following the 2005 Tarapaca earthquake in northern Chile. Despite the magnitude of the earthquake (7.9 on the Richter scale), the amount of destruction was relatively limited: eleven people died, and 0.035% of the population had to temporarily relocate to shelters. This was attributed to the low population density of the region as well as Chilean preparedness and building codes to withstand earthquakes. Although the damage was not as severe as it could have been, Torche reasoned that the earthquake likely caused acute maternal stress, which in turn could affect prenatal development. 

Looking at over half a million births, Torche used maternal county of residence as an estimate of the earthquake’s intensity across different trimesters of exposure. She found that mothers who were lived in the most intensely affected regions during the first trimester were the most affected. The probability of LBW increased from 4.7% to 6.5%, while rates of pre-term births also increased from 5.2% to 8.0%. Later periods of gestation were not substantially affected, and for infants who were conceived after the earthquake, the probability of LBW returned to baseline.

Again, these outcomes seemed to result primarily from acute psychological stress stemming from the earthquake. Torche reasoned that – given the relatively low amount of damage to infrastructure – the increases in LBW and pre-term births were unlikely to have resulted from other factors such as malnutrition, infection, stress resulting from deprivation, strenuous workloads, or exposure to environmental toxins. In reality, it’s not possible to control for all of these variables entirely, but overall it seems plausible that maternal psychological stress played a substantial role in birth outcomes.

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