Prenatal “Shocks” and Birth Outcomes


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.

Adversity of War

There isn’t much we can do about preventing earthquakes or other natural disasters, but it appears that human-made traumatic events can have similar effects on early growth as well.

Richard Steckel looked at historical birth weights during the late nineteenth and early twentieth centuries in a number of cities, including Vienna (Steckel 1998). Birth weight in that city typically fluctuated between 3,100 to 3,200 grams from 1865 to the turn of the century. During the First World War, average birth weight in Vienna dropped noticeably, after which it increased again. Unlike an earthquake – a short-lived event often followed by humanitarian aid – wars create a suite of chronic stressors, such as disrupted food supplies, spread of infectious disease, broken trade routes, forced displacement, and psychological stress from lingering fear and uncertainty. Therefore, it is difficult to attribute declines in birth weight to any single form of stress. The entirety of the war is a sustained shock or insult to prenatal (and postnatal) growth and development.

Steckel

From Steckel 1998.

 

Similarly, Clement Smith (1947) found that birth weights and conceptions declined in the Netherlands during World War II. This coincided with a punitive period of famine known as the Dutch Hunger Winter, imposed upon a handful of cities by occupying Nazi forces.

Smith reported that official rations for pregnant women in western Holland declined from 2,353 calories per day in December 1943 to as few as 1,144 calories by January 1945. After liberation by Allied forces in early May 1945, rations increased to 2,546 calories. In Leiden and The Hague, available daily calories for pregnant may have declined as low as 731 by February 1945. Micronutrients also declined.

Of course, official rations are not necessarily what people actually ate, but birth weight in the city of Rotterdam closely followed trends in available calories. To account for gestation length, Smith looked at infants who were born at forty weeks gestation and those who were born at thirty-eight weeks and over. In both categories, birth weights (n = 873) reached their nadir by April to June of 1945, after which they increased again.

smith-47-e1505520469827.jpg

From Smith, 1947. Distribution of birth weights during and after the hunger winter in Rotterdam (Oct 1 1944 to Dec 21 1945). Percentiles are on the left from prewar times.

 

Data from other war-affected nations show similar patterns. In the Soviet city of Kursk, average birth weight dropped about 150 grams from 1939 to 1944 (Vlastovsky 1966). In their 1950 two-volume work, The Biology of Human Starvation, Ancel Keys and colleagues reported that birth weight had dropped in several countries during WW2 including France, Greece, and Austria.

 

Other Stressors: Landmines, Conflict Fatalities, Homicides

Adriana Camacho (2008) looked at whether birth weights might be affected by a very specific aspect of military conflict: maternal exposure to landmine explosions in Colombia. To be clear, mothers were not directly injured by landmines. Rather, simple awareness of a landmine detonation in one’s department/ region appears to be sufficient enough to decrease birth weight. Similar to the effect of the earthquake in Chile, Camacho reasoned that proximity to a landmine would induce psychological stress in the mother, having downstream effects on their newborns. Looking at 4 million births in Colombia from 1998 to 2003, Camacho found that “children born in a departmento with at least one landmine explosion in each trimester of pregnancy weigh on average 27.76g less than those born with no explosions.” Furthermore, the effect seemed to be greatest in the first trimester.

This was consistent with other studies, including a working paper by Climent Quintana-Domeque and Pedro Rodenas (2014) which found that terrorist attacks by Basque separatists in Spain (ETA) had a negative effect on birth weight. Looking at over six million births, they found that “an additional ETA-bomb casualty in the first trimester of pregnancy (on average) decreases birth weight by around 0.7 grams, increases the expected number of low birth-weights by around 0.2 per 1,000 live births, and decreases the predicted number of normal deliveries (without pregnancy or labor complications) by about 0.6 per 1,000 live births.”

Likewise, Hani Mansour and Daniel Rees (2012) looked at the effects of lethal violence in Palestinian territory during the Second Intifada. Among 1,224 infants born between April 2001 and June 2004 they found that “an additional fatality 2-0 months before birth is associated with a decrease in birth weight of 2.1 grams,” a small but statistically significant effect. There was also a modest increase in the probability that an infant would be born LBW.

A similar study in Brazil looked at the effects of homicides on birth weight across neighborhoods in the city of Fortaleza, considered one of the most violent cities in the country (Koppensteiner and Manacorda 2016). They found that nearby homicides in the first trimester of pregnancy had a negative effect on birth weight, and this effect was greater in rural neighborhoods, where such events were rarer and more impactful. They estimated that homicide rates accounted “for around 1 percent of the incidence of low birthweight and 3.5 percent of the incidence of extreme low birthweight.”

koppensteiner

From Koppensteiner and Manacorda, 2016. Fraction of infants born low birth weight (left) and homicides in the public across neighborhoods of Fortaleza, Brazil.

 

Shocks from Discrimination

Studies that explore correlations between birth weight and other variables such as explosions, homicides, and conflict-related mortality certainly have their strengths: they are quantifiable and can be applied to large samples. However, their effects appear to be modest. Perhaps a limitation of such approaches is that they don’t necessarily grasp how individual mothers subjectively experience stressful events that could affect their fetuses. One way to ascertain this is to look at specific groups of people who might plausibly feel vulnerable or targeted by a given stressor.

Diane Lauderdale (2006) looked at whether perceived or real ethnic discrimination after the terrorist attacks of September 11, 2001 might have had an impact on birth weight among Arab-American women living in California. She found that before Sept 11, rates of LBW in Arab-American infants were not significantly different from white infants. By comparison, black, Asian, and Hispanic infants all had significantly higher odds of being born LBW than did whites.

However, in the six months after Sept 11, Arab-American women experienced a 34% elevated risk of having a LBW infant compared to before that date. This effect was even greater among infants who had “ethnically distinctive” given names (an indication that those mothers might have stronger ethnic identities), with the LBW risk more than doubling compared to rates before Sept 11. Rates for other ethnic groups did not change after that date. While Lauderdale was cautious in saying that the increasing rates of LBW in Arab-American infants was not direct evidence of ethnic discrimination, it seems fairly likely, given that rates changed so dramatically and in only one ethnic group. In fact, sustained psychological stress as a result of ethnic discrimination has been implicated as one possible cause of higher LBW rates found among African-American infants (David and Collins, 1997).

Furthermore, the effects of ethnic-specific stressors on birth weight are seen in a recent study conducted on a major government raid against migrant Latino workers in Postville, Iowa who were suspected of being undocumented (Novak et al 2017). In May of 2008, nearly 400 Latino workers were arrested at a meat-processing plant in Postville by 900 ICE agents in a surprise raid that used military tactics, including a Black Hawk helicopter. Two hundred and ninety-seven people were eventually deported after a five month prison sentence. However, even those who were not deported (or even arrested) were affected.

Novak et al. looked beyond Postville for their sample, including 52,344 births across Iowa from 2007 to 2009. Infants who were born in the 37 weeks following the raid were classified as being prenatally “exposed” to the raid’s effects. The results showed that for White and Latina mothers, rates of LBW were steady before the raid. However, after the events at Postville, LBW rates increased by 24%, though only among Latina mothers. This was true for both foreign-born and US born Latinas and their infants, showing a widespread effect that was dependent upon ethnicity regardless of immigration status. The effect appeared to be greatest among those mothers who were exposed to the raid in the first trimester.

novak

From Novak et al. 2017. Rates of LBW in three groups of mothers/infants in Iowa: Whites, U.S. born Latinas, and foreign-born Latinas.

 

Because data were only available at the state rather than local level, Novak et al. were unable to determine if rates of LBW were even higher among Latina mothers who lived closer to Postville than those in more distant parts of the state. However, the authors reasoned that Latina mothers throughout Iowa (and possibly outside of the state as well) were likely aware of the raid, either through following the news or direct communications with friends and relatives. Given that rates increased among U.S. born and foreign-born Latinas, it is certainly possible that Latina mothers felt vulnerable or anxious about raids targeting them for their ethnicity, and that the stress from this could trickle down to their infants.

Summary

Collectively, the above studies make a decent case that prenatal stressors/ insults/ shocks can affect birth weight, whether they results from natural disasters; war; nearby explosions, deaths, homicides; or ethnic discrimination. The concern is not merely with LBW itself. Rather, LBW and intrauterine growth restriction are risk factors for health problems later in life, per the developmental origins of health and disease (DOHaD) hypothesis, though that is another post unto itself. In short, prenatal adversity and developmental deficits early in life are a risk factor for health deficits later in life.  

It seems obvious that we would want to protect mothers and their fetuses from outside stressors to give them the best chance at a healthy pregnancy and a good early start in life. While natural disasters like earthquakes are beyond our ability to control, the other “shocks” described above are all anthropogenic. Therefore, they are within our power to curtail, with potential payoffs in terms of development and health.

 

References

Camacho A. 2008. Stress and birth weight: Evidence from terrorist attacks. American Economic Review, 98(2): 511–5. Link

David R, Collins J. 1997. Differing birth weight among infants of U.S.-born blacks, African born blacks, and U.S. born whites. New Engl J Med 337: 1209-14. Link

Keys A, Brožek J, Henschel A, Mickelsen O, Taylor HL. 1950. The Biology of Human Starvation (2 volumes), University of Minnesota Press.

Koppensteiner MF, Manacorda M. 2016. Violence and birth outcomes: Evidence from homicides in Brazil. Journal of Development Economics. 31;119:16-33. Link

Lauderdale DS. 2006. Birth outcomes for Arabic-named women in California before and after September 11. Demography. 43(1):185-201. Link

Mansour H, Rees DI. 2012. Armed conflict and birth weight: Evidence from the al-Aqsa Intifada. Journal of Development Economics. 99(1):190-9. Link

Novak NL, Geronimus AT, Martinez-Cardoso AM. 2017. Change in birth outcomes among infants born to Latina mothers after a major immigration raid. International Journal of Epidemiology. 46(3): 839-49. Link

Quintana-Domeque C, Rodenas P. 2014. Fear in the womb: the effects of terrorism on birth outcomes in Spain, mimeo. University of Oxford. Link

Smith CA. 1947. Effects of maternal undernutrition upon the newborn infants in Holland (1944-1945). J Pediatrics 30: 229-243.

Steckel RH. 1998.  Birth weights and stillbirths in historical perspective. European Journal of Clinical Nutrition 52:S16-S20. Link

Torche F. 2011. The effect of maternal stress on birth outcomes: exploiting a natural experiment. Demography. 48(4):1473-91. Link

Vlastovsky VG. 1966. The secular trend in the growth and development of children and young persons in the Soviet Union. Hum Biol 38(3):219-30.

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