Fetal origins hypothesis

The fetal origins hypothesis (differentiated from the developmental origins of health and disease hypothesis, which emphasizes environmental conditions both before and immediately after birth) proposes that the period of gestation has significant impacts on the developmental health and wellbeing outcomes for an individual ranging from infancy to adulthood. The effects of fetal origin are marked by three characteristics: latency, wherein effects may not be apparent until much later in life; persistency, whereby conditions resulting from a fetal effect continue to exist for a given individual; and genetic programming, which describes the 'switching on' of a specific gene due to prenatal environment. Research in the areas of economics, epidemiology, and epigenetics offer support for the hypothesis. The fetal origins hypothesis (differentiated from the developmental origins of health and disease hypothesis, which emphasizes environmental conditions both before and immediately after birth) proposes that the period of gestation has significant impacts on the developmental health and wellbeing outcomes for an individual ranging from infancy to adulthood. The effects of fetal origin are marked by three characteristics: latency, wherein effects may not be apparent until much later in life; persistency, whereby conditions resulting from a fetal effect continue to exist for a given individual; and genetic programming, which describes the 'switching on' of a specific gene due to prenatal environment. Research in the areas of economics, epidemiology, and epigenetics offer support for the hypothesis. The fetus was once believed to be a 'perfect parasite', immune to harmful environmental toxins passed from the mother via the placenta. Stemming from this belief, pregnant women of the early to mid 20th century freely drank alcohol, ingested medications, smoked cigarettes, and were largely ignorant of any nutritional needs for a developing fetus. This easy going attitude about pregnancy was challenged, however, by findings relating substances ingested by a mother to tragic outcomes for a fetus. The birth defects crisis due to the medication thalidomide in the 1960s, where thousands of children were born with defects ranging from brain damage to truncated and missing arms and legs is an example of how a seemingly miracle medication supposed to prevent morning sickness instead had disastrous consequences. Similarly, in 1971, a drug known as DES, diethylstilbestrol, when taken by pregnant women, was found to be causing an incredibly rare vaginal cancer known as clear-cell adenocarcinoma in young girls when the cancer was traditionally only found to affect those of post-menopausal age. This finding, in particular, demonstrates that events occurring during gestation are capable of impacting future health into adulthood. As perhaps the most well-known fetal risk, It wasn't until 1973 that fetal alcohol syndrome was first formally diagnosed, and not until 1989 that the United States government began requiring warning labels directed at pregnant women to be in place on all alcoholic beverages for sale. While the risks associated with certain substances have been well documented during pregnancy, the fetal origins hypothesis goes beyond medical substances to expand upon the effects of maternal stress, obesity, influenza, nutrition, and pollution on a developing fetus. Epidemiologist David Barker was the earliest proponent of the theory of fetal origins of adult disease, prompting the theory to be denoted as 'Barker's hypothesis'. In 1986, Barker published findings proposing a direct link between prenatal nutrition and late-onset coronary heart disease. He had noticed that the poorest areas of England were the same areas with the highest rates of heart disease, unearthing the predictive relationship between low birth weight and adult disease. His findings were met with criticism, mainly because at the time heart disease was considered to be predominantly determined by lifestyle and genetic factors. Since Barker's initial findings, the results have been replicated in diverse populations of Europe, Asia, North American, Africa, and Australia. In explanation of such findings, Barker suggests that fetuses learn to adapt to the environment they expect to enter into once outside of the womb. Essentially, all transmissions entering the placenta act as 'postcards' giving the fetus clues as to the outside world, preparing its physiology appropriately. This can be an adaptive mechanism, when fetal conditions accurately represent the world of birth; alternatively, it can be a harmful mechanism, when fetal conditions of plenitude or scarcity do not match the world of birth and the child has been physiologically predisposed to inhabit an environment where expected resources are drastically different from reality. The thrifty phenotype hypothesis proposes that a low availability of nutrients during the prenatal stage followed by an improvement in nutritional availability in early childhood causes an increase risk of metabolic disorders, including Type II diabetes, as a result of permanent changes in the metabolic processing of glucose-insulin determined in utero. This predominantly affects poor communities, where maternal malnutrition may be rampant, in turn causing fetuses to be biologically programmed to expect sparse nutritional environments. But, once in the world, the readily accessible processed foods consumed are unable to be processed efficiently by individuals who had their metabolic systems pre-set to expect scarcity. This difference between expected nutritional deficits and actual food surplus results in obesity and eventually Type II Diabetes. Janet Rich-Edwards, an epidemiologist at Harvard Medical School, initially set out to disprove the fetal origins theory with her database of over 100,000 nurses. Instead, she found that the results hold: a strong relationship exists between low birth weight and later coronary heart disease and stroke. Pregnancy outcomes can impact the wellbeing of a society. Comparisons between the children who were in gestation during the 1918 flu pandemic and those in gestation immediately before or after the health crisis show marked differences between the two groups on census data. Across all socioeconomic measures, those who were fetuses during the crisis attained lower educational achievement, income, and socioeconomic status. Specifically, individuals affected were 15% less likely to graduate high school, 15% more likely to be poor, and 20% more likely to be disabled as adults. Even federal welfare payments were higher for the gestational cohort than those born before or after the flu hit. The same economic researcher, Douglas Almond, has investigated other historical situations affecting particular cohorts of fetuses: children born during or immediately following the Chernobyl nuclear disaster explosion, and China's Great Leap Forward (which resulted in a deadly famine). Both prenatally exposed groups suffered lower cognitive abilities and reduced employment levels. Such outcomes can have lasting impacts on the productivity and economic security of a society for an entire generation of individuals, and perhaps even continue to affect future descendants through changes in gene expression. Epigenetics refers to the study of the behavior of genes, and how gene expression can be altered by the environment without changes made in DNA. This is believed to be particularly possible during prenatal development, and both stress and diet have been known to causes changes to a fetus. Findings linking maternal exposure to pollution with poor health outcomes for children are possibly linked to the altering of gene expression. Additionally, studies focusing on maternal weight show gene altering may be occurring. Women who are overweight at the time of pregnancy have children that are more likely to be overweight themselves. This could be due to the genetic heritability of genes related to obesity. But, siblings born to these same women after they had weight reduction surgery were no more likely to be overweight than the rest of the general population. The metabolic nature of the children was completely different, despite being born to the same mother, supporting the idea that the gestational environment strongly influences future outcomes. In discussing the epigenetics findings of fetal origins, Princeton University's Janet Currie says, 'The long-vaunted distinction between nature and nurture is therefore outdated and unhelpful. Poor nurture during pregnancy can worsen the hand that nature has dealt.'