Childbirth fear, which has been argued to have an adaptive basis, exists on a spectrum. Pathologically high levels of childbirth fear is a clinical condition called tokophobia. As a chronic stressor in pregnancy, tokophobia could impact birth outcomes. Many factors associated with tokophobia, including inadequate labor support, were exacerbated by the COVID-19 pandemic. We used longitudinally collected data from a convenience sample of 1775 pregnant persons in the USA to evaluate the association between general and COVID-19 pandemic-related factors and tokophobia using the fear of birth scale. We also assessed associations between tokophobia, low birth weight and preterm birth when adjusting for cesarean section and other covariates among a subset of participants (N = 993). Tokophobia was highly prevalent (62%). Mothers who self-identified as Black (odds ratio (OR) = 1.90), had lower income (OR = 1.39), had less education (OR = 1.37), had a high-risk pregnancy (OR = 1.65) or had prenatal depression (OR = 4.95) had significantly higher odds of tokophobia. Concerns about how COVID-19 could negatively affect maternal and infant health and birth experience were also associated with tokophobia (ORs from 1.51 to 1.79). Tokophobia was significantly associated with increased odds of giving birth preterm (OR = 1.93). Tokophobia increases the odds of preterm birth and is more prevalent among individuals who are Black, have a lower income, and have less education. Tokophobia may, therefore, be an underappreciated contributor to inequities in US birth outcomes. The COVID-19 pandemic likely compounded these effects.

Two hundred million people worldwide experience some form of thyroid disorder, with women being especially at risk. However, why human thyroid function varies between populations, individuals and across the lifespan has attracted little research to date. This limits our ability to evaluate the conditions under which patterns of variation in thyroid function are best understood as 'normal' or 'pathological'. In this review, we aim to spark interest in research aimed at understanding the causes of variation in thyroid phenotypes. We start by assessing the biomedical literature on thyroid imbalance to discuss the validity of existing reference intervals for diagnosis and treatment across individuals and populations. We then propose an evolutionary ecological framework for understanding the phylogenetic, genetic, ecological, developmental and physiological causes of normal variation in thyroid function. We build on this approach to suggest testable predictions for how environmental challenges interact with individual circumstances to influence the onset of thyroid disorders. We propose that dietary changes, ecological disruptions of co-evolutionary processes during pregnancy and with pathogens, emerging infections and exacerbated stress responses can contribute to explaining the onset of thyroid diseases. For patients to receive the best personalised care, research into the causes of thyroid variation at multiple levels is needed.