|How we are ushered into life determines how we leave.|
'Fetal programming', an important component of fetal development, refers to the real-time adaptive anatomical and physiological responses a fetus makes to the intrauterine environmental conditions it experiences while it is living, growing, and organizing itself in its mother's womb. The responses differ in character depending upon whether the environmental condition is beneficial or unfavorable to the viability of the fetus, and depending upon the precise nature of the environmental condition experienced by the fetus. The character of the responses are adaptive for fetal viability, consisting in setting (programming) structural, physiological, and metabolic features of the fetus, hence in the structural, physiological, and metabolic features of the its body after birth.
Thus a fetus's intrauterine environment contributes to the programming of its growth and developing self-organization. When a fetus experiences a suboptimal condition, such as failure of maternal supply of adequate nutrition, the fetus will grow and develop abnormally, resulting in a newborn infant with abnormal structural, metabolic, and physiological characteristics that can increase its susceptibility to disease in later life. [Note 1]
Despite its immature state, a fetus constitutes a living system, a living complex adaptive system. Because it continually grows and self-organizes, it can respond with high sensitivity to environmental conditions; its structural, physiological and metabolic states have a great degree of plasticity. It adapts in real-time to adverse environmental conditions that threaten its viability. Those adaptations might include slowing its growth rate, reducing the number of cells in its organs, altering metabolic pathways, and altering its physiological responses to normal stimuli. If those adaptations serve to maintain the life of the fetus, they will persist throughout the fetus's development and result in an abnormally functioning newborn, persisting throughout childhood, adolescence, and adulthood. If the threatening intrauterine conditions no longer continue after birth, the child or adult may no longer have the ability to adapt to the newer conditions, and as a result, become susceptible to the maladaptations that constitute disease. A fetus adapted to survive to suboptimal nutrition may, in later life, be unable to adapt to conditions of enriched nutrition, responding abnormally. [Note 1]
In effect, the fetus's adaptations 'program' the person it becomes to the responses it makes to its post-natal environment, as an infant and throughout the remainder of its lifetime.
The major, but not exclusive, environmental influences on the type and degree of fetal programming derive from the fetus's maternal connection via the placenta,  hence in part from the health status of the mother, both physical and mental.
In 2011, University of Columbia reseearchers, Zeltser and Leibel, emphasizing the role of the placenta, note:
Following on the seminal observations of Barker and associates ([cites:]), maternal hormonal and nutrient environment has been systematically implicated in effects on the developing fetus that ultimately influence susceptibility to a wide range of metabolic, neurodevelopmental, and psychiatric diseases in adulthood ([cites: ]). There is a growing appreciation that perturbations in the maternal environment are conveyed to the fetus by changes in placental function ([cites:]).
In a more recent review, psychoneuroendocrinologist Sonja Entringer describes fetal programming this way:
Substantial evidence in humans and animals suggests that conditions during intrauterine life play a major role in shaping not only all aspects of fetal development and birth outcomes but also subsequent newborn, child, and adult health outcomes and susceptibility for many of the complex, common disorders that confer the major burden of disease in society (i.e., the concept of fetal, or developmental, origins of health and disease risk) [cites:  ].
Focusing on pathophysiology, fetal programming also goes by the name, 'fetal origins of adult disease'. From a broader perspective than the pathophysiological, however, the fetus also responds to beneficial intrauterine environments, adapting its metabolism, physiology, and structure to health and lower susceptibility to disease in later life. For one example, in the studies of Barker mentioned above, the babies born with higher birth-weight due to more optimal maternal nutrition had significantly lower risk of developing coronary heart disease than did the lower birth-weight babies.
Recognition of fetal programming led to recognition that the earliest stages of development, including infancy, could respond to environmental conditions in ways that influenced health status in later life, which, in turn, led to a new discipline, The Developmental Origins of Health and Disease. [Note 3] 
Postnatal disease types sensitive to fetal programming
Adverse types of fetal environmental conditions promoting fetal programming
Maternal nutritional abnormalities
Paternal genetic abnormalities
Maternal hormonal abnormalities
Examples of fetal programming in humans
In 1986, David Barker and Clive Osmond reported on their studies of the relationships among infant mortality, childhood nutrition, and adult ischemic heart disease in England and Wales. By geographical regions, past infant mortality rates, highest where poverty was greatest, associated positively with present occurrences of ischemic heart disease, whereas increasing heart disease presently associated with increasing prosperity. From their analysis the investigators suggested that “poor nutrition in early life increases susceptibility to the effects of an affluent diet”.
Fetal programming applies also to age-related cognitive decline. A long term follow-up study in men by Katri Raikkonen and colleagues showed that lower cognitive ability at mean age 67.9 years associated with lower birth-weight, birth-length, and birth-head-circumference. Similarly, cognitive decline after age 20 years associated with those lower measures of intrauterine physical growth. The investigator found that in "predicting resilience to age related cognitive decline, the period before birth seems to be more critical" compared to the period of infancy.
Examples of fetal programming in non-human animals
In sheep, suboptimal maternal nutrition coincident with early fetal kidney development results in enhanced renal lipid deposition following juvenile obesity and could accelerate the onset of the adverse metabolic, rather than cardiovascular, symptoms accompanying the metabolic syndrome.
Fetal programming response to maternal stress
Reverse fetal programming: fetal programming of mother
References cited in text