Humans are not unique: difficult birth is common in placental mammals

Human childbirth is widely presumed to be uniquely difficult and dangerous compared to birth in other mammals. Tight fetopelvic proportions can result in obstructed labour and contribute to high rates of maternal and neonatal mortality. Ideas summarised under the 'obstetrical dilemma' have contributed to this assumption by explaining difficult childbirth as the outcome of a unique evolutionary history of bipedalism and encephalisation. However, a comprehensive review of the literature and an analysis of collated data demonstrate that human birth is not truly exceptional in its risk. Dystocia (birth difficulties) occurs across a phylogenetically wide range of placental mammals, including wild populations where natural selection would be expected to minimise its occurrence. Various species show similar or higher dystocia prevalence compared to humans, and fetopelvic disproportion is particularly common in species with precocial, large-bodied offspring. The underlying causes and risk factors are also highly comparable. Contrary to long-standing assumptions, natural selection has not eliminated dystocia in natural mammal populations, suggesting that parturition in placentals may be characterised by a persistent baseline risk of complications. This persistent risk likely reflects a life history trade-off between the improved survival of large offspring and the occasional costs of birth complications. A cliff-edge model of selection can explain the evolutionary dynamics underlying this trade-off and the persistent fraction of fetopelvic disproportion and related forms of obstructive dystocia. An additional trade-off involving litter size is evident in litter-bearing, often altricial species, where both small and large litters increase the risk of dystocia. Phenotypic plasticity further maintains dystocia in populations despite selection, while also contributing to variation in birth difficulty within and among species. These findings reposition human birth difficulties within a broader mammalian pattern rather than as a uniquely human phenomenon requiring exceptional explanations. Reframing human childbirth in this broader context challenges entrenched assumptions and highlights the value of a comparative evolutionary perspective.