What Was Human Birth Weight in the Past? Simulations Based on Data on Stature from the Palaeolithic to the Present

An Evolutionary Model of “Sexual Conflict” Over Women's Age at Marriage: Implications for Child Mortality and Undernutrition

Background

Early women's marriage is associated with adverse outcomes for mothers and their offspring, including reduced human capital and increased child undernutrition and mortality. Despite preventive efforts, it remains common in many populations and is often favored by cultural norms. A key question is why it remains common, given such penalties. Using an evolutionary perspective, a simple mathematical model was developed to explore women's optimal marriage age under different circumstances, if the sole aim were to maximize maternal or paternal lifetime reproductive fitness (surviving offspring).

Methods

The model was based on several assumptions, supported by empirical evidence, regarding relationships between women's marital age and parental and offspring outcomes. It assumes that later marriage promotes women's autonomy, enhancing control over fertility and childcare, but increases paternity uncertainty. Given these assumptions, optimal marriage ages for maximizing maternal and paternal fitness were calculated. The basic model was then used to simulate environmental changes or public health interventions, including shifts in child mortality, suppression of women's autonomy, or promoting women's contraception or education.

Results

In the basic model, paternal fitness is maximized at lower women's marriage age than is maternal fitness, with the paternal optimum worsening child undernutrition and mortality. A family planning intervention delays marriage age and reduces child mortality and undernutrition, at a cost to paternal but not maternal fitness. Reductions in child mortality favor earlier marriage but increase child undernutrition, whereas ecological shocks that increase child mortality favor later marriage but reduce fitness of both parents. An education intervention favors later marriage and reduces child mortality and undernutrition, but at a cost to paternal fitness. Efforts to suppress maternal autonomy substantially increase fitness of both parents, but only if other members of the household provide compensatory childcare.

Conclusion

Early women's marriage maximizes paternal fitness despite relatively high child mortality and undernutrition, by increasing fertility and reducing paternity uncertainty. This tension between the sexes over the optimal marriage age is sensitive to ecological stresses or interventions. Education interventions seem most likely to improve maternal and child outcomes, but may be resisted by males and their kin as they may reduce paternal fitness.

Pelvic and neonatal size correlations in light of evolutionary hypotheses

OBJECTIVE

This study aimed to analyze the correlations between maternal size, neonatal size, and gestational variables.

METHODS

Our sample comprises 131 mother-infant dyads. We investigated correlations between five neonatal traits (gestational age, birthweight, head, suboccipito-brematic, and abdominal girths), three maternal traits (height, BMI, and uterus height), and three pelvic variables (conjugate, inter-spinous diameters, and sub-pubic angle) using computed tomography pelvimetry.

RESULTS

We found that the five neonatal traits were significantly intercorrelated. BMI was not correlated with neonatal traits while maternal height was correlated with birthweight, suboccipito-brematic, and abdominal girth. In the multiple regression models, gestational age was correlated with birthweight, head, and abdominal girth. Among the neonatal and pelvimetry correlations, conjugate diameter was slightly correlated with suboccipito-bregmatic girth, but inter-spinous and sub-pubic angle were not correlated with neonatal traits. Uterus height predicted all neonatal variables, but it was not correlated with gestational age.

DISCUSSION

Our results suggest that fetal growth is shaped by maternal phenotype rather than external ecological factors. The association of the inlet size with suboccipito-bregmatic girth reflects the tight fit between the neonatal brain and the maternal pelvis dimensions, an adaptation that would reduce the risk of cephalo-pelvic disproportion, while the absence of tight fit at the midplane and outlet could be due to the effect of the pelvic relaxation. Uterus distention is not the only mechanism involved in the initiation of parturition. Birth and pregnancy are complex processes and we suggest that maternal-neonatal associations are the result of a combination of multiple obstetric tradeoffs.

Secular changes in body height predict global rates of caesarean section

The massive global variation in caesarean-section (C-section) rate is usually attributed to socio-economic, medical and cultural heterogeneity. Here, we show that a third of the global variance in current national C-section rate can be explained by the trends of adult body height from the 1970s to the 1990s. In many countries, living conditions have continually improved during the last century, which has led to an increase in both fetal and adult average body size. As the fetus is one generation ahead of the mother, the fetus is likely to experience better environmental conditions during development than the mother did, causing a disproportionately large fetus and an increased risk of obstructed labour. A structural equation model revealed that socio-economic development and access to healthcare affect C-section rate via multiple causal pathways, but the strongest direct effect on C-section rate was body height change. These results indicate that the historical trajectory of socio-economic development affects-via its influence on pre- and postnatal growth-the intergenerational relationship between maternal and fetal dimensions and thus the difficulty of labour. This sheds new light on historic and prehistoric transitions of childbirth and questions the World Health Organization (WHO) suggestion for a global 'ideal' C-section rate.

Could consanguineous marriage provide a cultural alleviation for the obstetric dilemma?

In placental mammals, a poor fit between the physical dimensions of the fetus and maternal pelvis increases the likelihood of obstructed labour. This problem is especially relevant to humans, as our species demonstrates both unique adaptations in pelvic shape and structure associated with bipedalism, and fetal encephalization. Natural selection is expected to have favoured adaptations that reduce the chances of such mismatch within individual mother-offspring dyads. Here, I hypothesise that the cultural practice of consanguineous marriage may have been favoured, on account of increasing the genetic similarity between mothers and offspring and hence the correlation between maternal and fetal physical dimensions. These benefits could be amplified if consanguineous marriage was accompanied by assortative mating for height. An additional benefit of consanguineous marriage for childbirth is the slight reduction in birth size of such offspring compared to non-consanguineous unions. Although the offspring of consanguineous unions have elevated risks of morbidity and mortality, these risks are moderate and the practice could still have been favoured by selection if the reduction in maternal mortality was greater than the increased mortality among individual offspring. This hypothesis could be tested directly by investigating whether rates of obstructed labour are lower in individuals and populations practising consanguineous marriage. At a broader level, phylogenetic analysis could be conducted to test whether consanguineous marriage appears to have originated in the areas where intensive agriculture was first practiced, as adult height typically fell in such populations, potentially exacerbating the risk of obstructed labour.

The diabesity epidemic in the light of evolution: insights from the capacity-load model

The global nutrition transition, which embraces major changes in how food is produced, distributed and consumed, is associated with rapid increases in the prevalence of obesity, but the implications for diabetes differ between populations. A simple conceptual model treats diabetes risk as the function of two interacting traits: 'metabolic capacity,' which promotes glucose homeostasis, and 'metabolic load', which challenges glucose homoeostasis. Population variability in diabetes prevalence is consistent with this conceptual model, indicating that the effect of obesity varies by ethnicity. Evolutionary life history theory can help explain why variability in metabolic capacity and metabolic load emerges. At the species level (hominin evolution), across human populations and within individual life courses, phenotypic variability emerges under selective pressure to maximise reproductive fitness rather than metabolic health. Those exposed to adverse environments may express or develop several metabolic traits that are individually beneficial for reproductive fitness, but which cumulatively increase diabetes risk. Public health interventions can help promote metabolic capacity, but there are limits to the benefits that can emerge within a single generation. This means that efforts to curb metabolic load (obesity, unhealthy lifestyles) must remain at the forefront of diabetes prevention. Such efforts should go beyond individuals and target the broader food system and socioeconomic factors, in order to maximise their efficacy.

Secular Dynamics of Body Height and Weight in Russian Children Aged 0–17

This study deals with long-term temporal changes of body height and weight during various stages of ontogeny: newborns, infancy, early age, fi rst childhood, second childhood, adolescence, and youth. Each age/sex group numbers ca 100 persons, the total sample size is ca 2 000. The meta-analysis is based primarily on growth standards for Russian children, regularly renewed by the Research Institute for the Hygiene and Health Protection of Children and Adolescents and mostly relating to separate decades of the 20th century. The intensity of the secular trends was assessed through the analysis of scatter plots. The largest share in the secular increase of bodily dimensions belongs to intense growth during the second year of life and during the adolescent growth spurt. The smallest share is that of intrauterine growth, limited by the mother’s body size, and that of growth during adolescence, when the mature body size has been virtually reached and growth rate is minimal. Boys, who are more eco-sensitive, demonstrate greater secular changes than girls, who are eco-resistant. Smaller secular changes in weight than in height in both boys and girls result in the increase of leptosomy. This heterochrony concerns mostly newborns, whose body mass is a standard example of stabilizing selection.

The influence of maternal height on offspring's birth weight in Merida, Mexico

OBJECTIVES

To analyze the influence of maternal height (MH) on offspring's birth weight (BW) in infants born from January 2016 to May 2017, in Merida, Mexico.

METHODS

A total of 3462 singletons born at term in a public hospital were analyzed. Data extracted from hospital records were MH; number of previous births; and infant's sex, BW, and gestational length. MH was divided into 4 categories at quartile points: Q1:129-147, Q2:148-151, Q3:152-155, and Q4:156-180 cm. BW data were transformed to z-score values using the WHO Child Growth Standard. The association between MH and infants' BW was analyzed through a multiple regression model in which gestational length, infant's sex, maternal age, ethnicity, and number of previous birth were included as covariates.

RESULTS

Mean values of MH and infants' BW were 151.43 cm (SD = 6.32) and 3172 g (SD = 401) (corresponding to -0.29 SD in BW z-score), respectively. The regression model showed that the shortest quartile of MH was associated with -0.43 (95% CI -0.35 -0.51) SD in infants' BW compared with the highest quartile. The second and third shortest quartiles of MH were associated with -0.24 (95% CI -0.16 -0.32) and -0.17 (95% CI -0.09 -0.25) SD of infants BW, respectively.

CONCLUSIONS

MH remains an important factor for infant BW even in the presence of other determinants. The influence of MH on infants' BW in this population is similar to that found in other populations in low-, middle-, and high-income countries.

The Dual Burden of Malnutrition Increases the Risk of Cesarean Delivery: Evidence From India

Background: Among contemporary human populations, rates of cesarean delivery vary substantially, making it difficult to know if the procedure is inadequately available, or used excessively relative to medical need. A much-cited evolutionary hypothesis attributed birth complications to an “obstetric dilemma,” resulting from antagonistic selective pressures acting on maternal pelvic dimensions and fetal brain growth during hominin evolution. However, the childbirth challenges experienced by living humans may not be representative of those in the past, and may vary in association with trends in ecological conditions. We hypothesized that variability in maternal phenotype (height and nutritional status) may contribute to the risk of cesarean delivery. In many populations, high levels of child stunting contribute to a high frequency of short adult stature, while obesity is also becoming more common. The combination of short maternal stature and maternal overweight or obesity may substantially increase the risk of cesarean delivery.

Methods: Using data from two large Indian health surveys from 2005–6 to 2015–2016, we tested associations of maternal somatic phenotype (short stature, overweight) with the risk of cesarean delivery, adjusting for confounding factors such as maternal age, birth order, rural/urban location, wealth and offspring sex.

Results: Secular trends in maternal body mass index between surveys were greater than trends in height. Maternal short stature and overweight both increased the risk of cesarean delivery, most strongly when jointly present within individual women. These associations were independent of birth order, wealth, maternal age and rural/urban location. Secular trends in maternal phenotype explained 18% of the increase in cesarean rate over 10 years.

Conclusion: Our results highlight how the emerging dual burden of malnutrition (persisting short adult stature which reflects persistent child stunting; increasing overweight in adults) is likely to impact childbirth in low and middle-income countries.

Maternal pelvic dimensions and neonatal size: Implications for growth plasticity in early life as adaptation

Patterns of fetal growth predict non-communicable disease risk in adult life, but fetal growth variability appears to have a relatively weak association with maternal nutritional dynamics during pregnancy. This challenges the interpretation of fetal growth variability as 'adaptation'. We hypothesized that associations of maternal size and nutritional status with neonatal size are mediated by the dimensions of the maternal pelvis. We analysed data on maternal height, body mass index (BMI) and pelvic dimensions (conjugate, inter-spinous and inter-cristal diameters) and neonatal gestational age, weight, length, thorax girth and head girth (n = 224). Multiple regression analysis was used to identify independent maternal predictors of neonatal size, and the mediating role of neonatal head girth in these associations. Pelvic dimensions displaced maternal BMI as a predictor of birth weight, explaining 11.6% of the variance. Maternal conjugate and inter-spinous diameters predicted neonatal length, thorax girth and head girth, whereas inter-cristal diameter only predicted neonatal length. Associations of pelvic dimensions with birth length, but not birth weight, were mediated by neonatal head girth. Pelvic dimensions predicted neonatal size better than maternal BMI, and these associations were mostly independent of maternal height. Sensitivity of fetal growth to pelvic dimensions reduces the risk of cephalo-pelvic disproportion, potentially a strong selective pressure during secular trends in height. Selection on fetal adaptation to relatively inflexible components of maternal phenotype, rather than directly to external ecological conditions, may help explain high levels of growth plasticity during late fetal life and early infancy.

The Elevated Susceptibility to Diabetes in India: An Evolutionary Perspective

India has rapidly become a "diabetes capital" of the world, despite maintaining high rates of under-nutrition. Indians develop diabetes at younger age and at lower body weights than other populations. Here, we interpret these characteristics in terms of a "capacity-load" model of glucose homeostasis. Specifically, we assume that glycemic control depends on whether the body's "metabolic capacity," referring to traits, such as pancreatic insulin production and muscle glucose clearance, is able to resolve the "metabolic load" generated by high levels of body fat, high dietary glycemic load, and sedentary behavior. We employ data from modern cohorts to support the model and the interpretation that elevated diabetic risk among Indian populations results from the high metabolic load imposed by westernized lifestyles acting on a baseline of low metabolic capacity. We attribute this low metabolic capacity to the low birth weight characteristic of Indian populations, which is associated with short stature and low lean mass in adult life. Using stature as a marker of metabolic capacity, we review archeological and historical evidence to highlight long-term declines in Indian stature associated with adaptation to several ecological stresses. Underlying causes may include increasing population density following the emergence of agriculture, the spread of vegetarian diets, regular famines induced by monsoon failure, and the undermining of agricultural security during the colonial period. The reduced growth and thin physique that characterize Indian populations elevate susceptibility to truncal obesity, and increase the metabolic penalties arising from sedentary behavior and high glycemic diets. Improving metabolic capacity may require multiple generations; in the meantime, efforts to reduce the metabolic load will help ameliorate the situation.

The obstetric dilemma: an ancient game of Russian roulette, or a variable dilemma sensitive to ecology?

The difficult birth process of humans, often described as the "obstetric dilemma," is commonly assumed to reflect antagonistic selective pressures favoring neonatal encephalization and maternal bipedal locomotion. However, cephalo-pelvic disproportion is not exclusive to humans, and is present in some primate species of smaller body size. The fossil record indicates mosaic evolution of the obstetric dilemma, involving a number of different evolutionary processes, and it appears to have shifted in magnitude between Australopithecus, Pleistocene Homo, and recent human populations. Most attention to date has focused on its generic nature, rather than on its variability between populations. We re-evaluate the nature of the human obstetric dilemma using updated hominin and primate literature, and then consider the contribution of phenotypic plasticity to variability in its magnitude. Both maternal pelvic dimensions and fetal growth patterns are sensitive to ecological factors such as diet and the thermal environment. Neonatal head girth has low plasticity, whereas neonatal mass and maternal stature have higher plasticity. Secular trends in body size may therefore exacerbate or decrease the obstetric dilemma. The emergence of agriculture may have exacerbated the dilemma, by decreasing maternal stature and increasing neonatal growth and adiposity due to dietary shifts. Paleodemographic comparisons between foragers and agriculturalists suggest that foragers have considerably lower rates of perinatal mortality. In contemporary populations, maternal stature remains strongly associated with perinatal mortality in many populations. Long-term improvements in nutrition across future generations may relieve the dilemma, but in the meantime, variability in its magnitude is likely to persist.