Abstract
Human preterm birth (PTB), a multifactorial syndrome affecting offspring born before 37 completed weeks of gestation, is the leading cause of newborn death worldwide. Remarkably, the degree to which early parturition contributes to mortality in other placental mammals remains unclear. To gain insights on whether PTB is a human-specific syndrome, we examined within- and between-species variation in gestation length across placental mammals and the impact of early parturition on offspring fitness. Within species, gestation length is normally distributed, and all species appear to occasionally give birth before the ‘optimal’ time. Furthermore, human gestation length, like that of many mammalian species, scales proportionally to body mass, suggesting that this trait, like many others, is constrained by body size. Premature humans suffer from numerous cognitive impairments, but little is known of cognitive impairments in other placental mammals. Human gestation differs in the timing of the ‘brain growth spurt’, where unlike many mammals, including closely related primates, the trajectory of human brain growth directly overlaps with the parturition time window. Thus, although all mammals experience early parturition, the fitness costs imposed by the cognitive impairments may be unique to our species. Describing PTB broadly in mammals opens avenues for comparative studies on the physiological and genetic regulators of birth timing as well as the development of new mammalian models of the disease.
Highlights
Preterm birth (PTB), defined in humans as birth before 37 completed weeks of gestation, is a complex multifactorial syndrome that originates when the complex interplay of ‘anatomical, physiological, biochemical, endocrinological and immunological events’ necessary for parturition is disrupted [1]
If mammals broadly experience PTB, what is the impact of variation in gestation length on offspring fitness across the species that exhibit it? In this critical review, we address the first question by surveying the distributions of gestational lengths and the second question through examining the evolutionary constraints on gestation lengths relative to body size and brain size across a wide range of placental mammals
To address the third question, we examine the fitness costs associated with early parturition in humans and other mammals
Summary
Preterm birth (PTB), defined in humans as birth before 37 completed weeks of gestation, is a complex multifactorial syndrome that originates when the complex interplay of ‘anatomical, physiological, biochemical, endocrinological and immunological events’ necessary for parturition is disrupted [1]. A complete understanding of PTB will require identifying the molecular and genetic mechanisms that control gestation length, pregnancy maintenance and initiation of parturition. The study of animal models has yielded many insights into the physiology of pregnancy, the functional mechanisms that result in early parturition remain poorly understood [7,8,9]. This is so in part to our lack of understanding of whether the syndrome of PTB is human-specific, or whether other organisms experience PTB as well. We discuss how the answers to these questions provide a novel evolutionary perspective to studying the molecular basis of PTB broadly in mammals
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