Epigenetics is a branch of genetics that studies the phenotypic changes of an individual (that is, their physical characteristics) that are not linked to changes in the individual's genetic makeup (that is, in the set of genes contained in the DNA of their cells). A natural question arises: what, then, causes these changes? The answer is that they are due to the regulation of the expression of these same genes. To understand what we're talking about, just picture a dimmer switch that controls the brightness of a light bulb: the bulb represents the gene, and the light it gives off is the effect the gene has on a particular physical trait; the more light there is, the greater the gene's effect on that trait. The switch in our metaphor is an epigenetic modification that regulates the intensity of the light given off by the bulb, and can even turn it off completely.
This regulatory mechanism, driven by epigenetic modifications of specific genes, can be short-term and dynamic, switching our light bulb on and off depending on whether light is needed, or it can be long-lasting and stable, switching our light bulb, and therefore the gene, on or off permanently (often only within the cells belonging to a particular tissue).
In living organisms, epigenetics plays a key role because it not only establishes which genes must show their effect and which must not during the various stages of life, but also determines the precise moment in development when these genes must be expressed. In human beings, in fact, all the cells of the embryo carry the same genetic information contained in the DNA, but not all genes will be useful to the individual at the same time or in the same tissues.
Pregnancy is one of the conditions in which epigenetics plays a key role. In this condition, the interaction between epigenetics and hormonal changes is essential for the tight control of the entire process of fetal development, from the earliest stages of gestation through to birth, which must occur only once the baby's development has been fully completed.
Let's now look at some stages of pregnancy in which the role of epigenetics has been demonstrated by various studies. Starting as early as ovulation, a large number of epigenetic changes take place that are responsible for the growth of the oocyte. After the mature oocyte is released and fertilized by the sperm cell, the zygote undergoes developmental changes that translate into the epigenetic reprogramming of some of the genes in its genome. This brings the genome into a state of gene expression compatible with totipotency (the ability of a cell to form all the cell types in the body) before implantation.
Most fetal development then takes place during the first trimester of pregnancy, when numerous epigenetic modifications occur in the embryo's genome. The way these changes specifically shape the development of the fetus's various tissues and structures is still under study today. One example is the formation of the neural tube (the structure from which the central nervous system will originate). This is a very important event in the development of the embryo, since incomplete development of this structure can lead to malformations known as «neural tube closure defects». Insufficient folic acid intake by the mother-to-be has long been linked to these events, and it has indeed been shown that a diet low in folic acid could alter the epigenetic modifications that are essential to prevent these malformations.
Throughout pregnancy, the mother's nutrition plays a fundamental role in fetal development and, later, in the child's development. It has in fact been shown that maternal nutrition has an effect on determining obesity in offspring in adulthood. These effects have been linked to epigenetic modifications of certain genes related to individual growth and to gene expression at the placental level.
As pregnancy progresses, the physical changes in the woman's body become increasingly evident. For example, breast tissue changes in preparation for breastfeeding, a process largely supported by lactogenic hormones secreted by the anterior pituitary gland. Numerous studies have shown that these changes are mediated, at least in part, by epigenetic mechanisms. Levels of epigenetic modification of genes related to milk production are in fact low in lactating mammary glands, allowing the expression of the genes involved in breastfeeding, while they are very high in the breast tissue of a woman who is not breastfeeding or in other tissues not involved in breastfeeding, where these genes are switched off entirely because they are not needed.
These are just examples that show how our body is programmed down to the smallest detail to carry out its biological functions. Given that we are dealing with such delicate mechanisms, it is easy to understand why pregnancy can sometimes be critical, both physically and psychologically. Indeed, everything that happens naturally and mechanically at the biological level can be influenced by the individual's psychological, mainly unconscious, dimension.
Source: The Epigenetics of Normal Pregnancy, Jonathan D Best, Nessa Carey – Obstet Med. 2013 Mar; 6(1): 3–7.