Selection in utero and population health: Theory and typology of research

Abstract

Public health researchers may assume, based on the fetal origins literature, that "scarring" of birth cohorts describes the population response to modern-day stressors. We contend, based on extensive literature concerned with selection in utero, that this assumption remains questionable. At least a third and likely many more of human conceptions fail to yield a live birth. Those that survive to birth, moreover, do not represent their conception cohort. Increasing data availability has led to an improved understanding of selection in utero and its implications for population health. The literature describing selection in utero, however, receives relatively little attention from social scientists. We aim to draw attention to the rich theoretical and empirical literature on selection in utero by offering a typology that organizes this diverse work along dimensions we think important, if not familiar, to those studying population health. We further use the typology to identify important gaps in the literature. This work should interest social scientists for two reasons. First, phenomena of broad scholarly interest (i.e., social connectivity, bereavement) affect the extent and timing of selection in utero. Second, the life-course health of a cohort depends in part on the strength of such selection. We conclude by identifying new research directions and with a reconciliation of the apparent contradiction between the "fetal origins" literature and that describing selection in utero.

Keywords: Cohort selection; Evolutionary theory; Life course health; Population stressors; Pregnancy loss; Reproductive suppression.

Fig. 1

Schematic illustration of fetal needs and maternal capacity to invest. TOP PANEL: X-axis describes Darwinian fitness. Y-axis on the left (orange) indicates level of resource need for fetuses F, which varies across pregnancies. Needs of fetuses F₁ decline with increased fitness. Y-axis on the right (blue dash) indicates maternal capacity to invest in children M₁; maternal capacity varies positively with maternal fitness. Selection in utero occurs when F₁ > M₁. Vertical dashed line indicates point at which F₁ = M₁. BOTTOM PANEL: To the left of this dashed line, the “left tail” of the frequency distribution of fetuses (shaded) undergoes selection in utero. Theory suggests that fewer high need infants will be born when the environment weakens women of reproductive age (i.e., down-shift of M₁ to M₂ while F₁ remains fixed). Selection in utero, moreover, would increase when maternal resources remain fixed but needs of the fetus shift upward (i.e., F₁ to F_2, with M₁ fixed). Perturbations of maternal capacity and fetal need may also occur simultaneously and interact according to parental-offspring conflict theory (Haig, 1993).

Fig. 2

Schematic illustration of selection in utero and fetal origins “damage” in response to a stressful environment. The solid curve shows the frequency distribution of fetal survivability in an average ambient environment. The vertical solid line represents the criterion below which a mother spontaneously terminates the pregnancy. An elevated stress-level environment, in which the maternal capacity to invest declines and the criterion for termination therefore shifts right, would increase selection in utero. Alternatively, in an elevated stress environment; the survivability distribution for fetuses may shift left, resulting in the dashed-line frequency distribution and cohort “damage” among the surviving births. In response to population stressors, both selection and damage in utero may occur but at different quantiles of the survivability distribution. In addition, the maternal criterion for spontaneous termination likely varies across the population, which suggests that selection and damage may be observed in the same conception cohorts.