Abstract
Evolutionary perspectives on human childbirth have primarily focused on characteristics of our species in general, rather than variability within and between contemporary populations. We use an evolutionary framework to explore how physical and demographic characteristics of mothers shape the risks of childbirth complications in rural lowland Nepal, where childbearing typically commences in adolescence and chronic undernutrition is widespread, though maternal overweight is increasing in association with nutrition transition. We conducted secondary analyses of data from a cluster-randomized trial. Women aged 14-35 years were categorized by age, number of previous pregnancies, height, body mass index (BMI), husband's education, and household wealth. Multivariable logistic regression models tested whether these characteristics independently predicted risks of episiotomy and cesarean section (CS, n= 14 261), and obstructed labor (OL, n= 5185). Risks were greatest among first-time adolescent mothers, though associations with age varied by outcome. Independent of age and parity, short stature and high BMI increased risks of CS and OL, whereas associations were weaker for episiotomy. Male offspring had increased risk of CS and OL but not episiotomy. Wealth was not associated with OL, but lower wealth and lower husband's education were associated with lower likelihood of episiotomy and CS. At the individual level, the risk childbirth complications is shaped by trade-offs between fertility, growth, and survival. Some biological markers of disadvantage (early childbearing, short stature) increased the risk, whereas low socio-economic status was associated with lower risk, indicating reduced access to relevant facilities. Independent of these associations, maternal age showed complex effects.
Highlights
From an evolutionary perspective, anthropologists have long approached the challenges of childbirth through the lens of the “obstetrical dilemma,” a term coined by Sherwood Washburn (Washburn, 1960)
All married women aged 10–49 years residing in 80 Village Development Committees (VDC) in the two selected districts who had not had operative family planning, and whose husbands had not had vasectomy were eligible to participate in the trial
When the three components of neonatal size were added to the model, birth weight was a significant predictor: each additional kilogram increased the odds of obstructed labor nearly 2.5 times (OR = 2.42, 95% CI 1.35, 4.35), there was little change in the OR for maternal body mass index (BMI), indicating that the association of maternal BMI was not mediated by neonatal size (Table S8)
Summary
Anthropologists have long approached the challenges of childbirth through the lens of the “obstetrical dilemma,” a term coined by Sherwood Washburn (Washburn, 1960) According to this approach, human childbirth is subject to antagonistic selective pressures favoring reduced size of the bony birth canal to support bipedal locomotion, but larger dimensions of the fetal head. These authors have argued that the human obstetrical dilemma has been considered too much of a “given,” and that women's pelvic variability is unrelated to the efficiency of locomotion (Warrener et al, 2015), that the short duration of pregnancy is linked with maternal energetic constraints on fetal growth (Dunsworth et al, 2012), that physical dimensions relevant to childbirth vary in association with local ecological conditions (Wells et al, 2012), and that childbirth complications could be reduced if traditional birthing positions and social strategies were more widely used (Rosenberg & Trevathan, 2018; Stone, 2016)
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