Blog originally posted at Global Health Hub!
In a previous entry on this site, I argued that herencia (“inheritance”) cannot explain the extremely high rates of child stunting observed in rural Guatemala. Using archaeologic, epidemiologic, and anthropologic evidence, I concluded that Maya children in rural Guatemalan are short because of social pathologies, rather than their genes.
In this post, I will extend a similar argument within the framework of fetal growth and newborn size.
Worldwide, large regional differences in the size of newborns have been observed among population groups. For example, one study estimated that the proportion of newborns “small for gestational age” (SGA) was almost 50% in Pakistan and India but only 6.5% in China. In total, more than 30 million children in low-income and middle-income countries were estimated to have been born SGA in 2010. Fetal and newborn growth matters as a risk factor for child mortality, as a proxy for maternal nutrition and health, and a predictor of future stunting .
In rural Guatemala, data on the size of newborns in is difficult to ascertain at a community or population level: The majority of births occur in the home, weight and height at birth as reported by parents or lay midwives are not reliable, and newborns may not present to health facilities for weeks or months after birth if a clinical problem does not arise.
However, two INCAP studies carried out in Guatemala since the 1950s have documented the disproportional number of infants born with low birthweight and length, even when controlling for preterm births. “INCAP’s research showed that [Guatemalans] become short adults by being born small and by growing poorly in early childhood,” an INCAP researcher summarized. More recently, in 2013, investigators utilizing the Global Network Maternal and Newborn Health Registry in Guatemala published data showing that a cohort of Maya term newborns were quite small (mean length-for-age Z-score −1.0 ± 1.01 and mean weight-for-age Z-score −0.88 ± 0.94).
These and other similar studies have shaped the ideology of the 1000 Days movement, which views child malnutrition as taking root at conception. Neverthless, it has remained an open question whether populations like the Maya are born small because of genetic factors, environmental factors, or a combination of factors.
A new seminal study
In this context, a terrific article by Villar et al. was published this month offering strong evidence that when mothers are healthy, there is very little difference in fetal and newborn size across diverse populations. Readers of this blog already may be familiar with this study, which was part of the INTERGROWTH-21st Project based at Oxford and was sponsored by the Bill & Melinda Gates Foundation. Science Daily offers a nice synthesis of the research conclusions:
It has previously been suggested that “race” and “ethnicity” are largely responsible for differences in the size of babies born in different populations and countries. These new results show that race and ethnicity are not the primary factors. What matters more is the educational, health and nutritional status of the mothers, and care provided during pregnancy.
Further details of the study are worth mentioning. The investigators measured various fetal and newborn attributes in two distinct cohorts in more than 20,000 healthy, affluent pregnant women in eight study sites: Pelotas (Brazil), Turin (Italy), Muscat (Oman), Oxford (UK), Seattle (USA), Beijing (China), Nagpur (India), and Nairobi (Kenya). Primary markers of fetal growth were fetal crown-rump length, fetal head circumference, and newborn size.
The following figure shows how across these populations only very small differences in average birthlength were found. In total, “only between 1.9% and 3.5% of the total variability in fetal skeletal growth and newborn length could be attributed to between-site differences.”
A further benefit of this study is that it allows for the construction of universal fetal and newborn growth standards across gestational ages; previously, a major challenge in estimating the burden of small for gestational age newborns has been the lack of universal reference standards.
Figure 4(C) from the paper: Birthlength standards at the 3rd, 50th, and 97th centiles. The different colors represent sensitivity analyses; red is the pooled curve.
The new fetal and newborn standards proposed are complementary to the child growth curves developed by the WHO in 2006. In short, together with the WHO child growth curves, we now have reliable growth standards from conception to birth to age 5.
“When mothers are in good health, babies grow in the womb in very similar ways the world over,” says Zulfiqar Bhutta. As an implementor of nutrition projects in rural Guatemala, I interpret this study’s findings as yet another piece of evidence that political, environmental, and economic explanations underlie chronic malnutrition here. Just as Paul Farmer has argued for infectious diseases in Haiti, stunting in Guatemala is the biologic expression of social inequalities.
The 1000 Days movement thus has appropriately focused attention on the issue of maternal health and nutrition during pregnancy. At the same time, we must be humble about the effects we expect to observe from the recommended package of technical interventions for pregnant mothers such as micronutrient supplementation, iron supplementation, and energy-protein provision. These are indeed evidence-based interventions, but their effect sizes are relatively modest and they alone cannot unwind the complex, noxious social realities from which malnutrition arises.
1. The terms used to categorize fetal and newborn growth are not always obvious. Fetal growth restriction (FGR), also known as intrauterine growth restriction (IUGR), is the term used to describe a fetus that has not reached growth potential, commonly defined in practice as less than the 10th percentile of fetal weight for gestational age as estimated by ultrasound. Small for gestational age (SGA) is an analagous term that refers to an newborn with weight lower than the 10th percentile for gestational age. Low birth weight (LBW) is a related term usually referring to any infants born at a weight of less than 2500 grams, regardless of gestational age. Thus, a preterm infant may have low birth weight but may not necessarily be growth restricted. In the literature, birth weights are more frequently reported than birth length, though it is length that is most pertinent in places with high rates of stunting.