The consequences of being born small - an adaptive perspective.

Absolute definitions of fetal growth are being replaced by definitions that focus on an optimal life-course trajectory. The fetus makes responses to its environment that are determined by the maternal macro-environment, health and physiology. The processes of maternal constraint create significant variations within the normal range of maternal environments and function, and in the fetal environment, which are reflected in different patterns of growth. Deficient nutrient provision may induce immediate adaptation in the form of fetal growth impairment, but will also induce adaptive responses that have evolved for predictive advantage; that is, for a later phase of the life cycle. This latter class of response, probably mediated by epigenetic processes, explains many outcomes of a less-than-optimal pregnancy, including impaired growth, increased visceral obesity, impaired cognitive development, advanced maturation and a greater risk of metabolic and related disease in later life. While these adaptive processes evolved and were appropriate in the environments of prehistory, they are increasingly mismatched with modern environments. Such considerations suggest different approaches to intervention and prevention in population-specific contexts.