Skip to main content
. 2023 Dec 4;131(12):126001. doi: 10.1289/EHP12956

Figure 1.

Figure 1A is a schematic illustration depicting the sources and routes of exposure during pregnancy for exposures associated with D N S methylation differences. Inhalation, ingestion, dermal, and absorption in a pregnant woman include diet and supplements; chemical contaminants in air, soil, and water; smoking and alcohol behaviors; and stress. Figure 1B is a schematic illustration depicting the methods for estimating participant exposure in perinatal environmental epigenome-wide association studies. There are two steps. Step 1: Data sources for exposure information include participant questionnaires, personal monitors, outdoor or satellite monitors, and spatial modeling; external doses include environmental sampling and chemical measures; and internal doses include biologic sampling and chemical or metabolite measures. Step 2: Data sources for exposure information, external dose, and internal dose lead to exposure assessment that includes quantifying levels of each exposure per participant and per time period. Figure 1C is a schematic illustration depicting the epidemiological frameworks for epigenome-wide association studies. There are three steps. Step 1: The epigenome-wide association study (E W A S): discovery of D N A methylation sites associated with exposures. The exposure and D N A methylation are interconnected. Step 2: Causation: Determining exposures cause D N A methylation changes. Requires additional assumptions and study design considerations. The exposure leads to D N A. Step 3: Reverse causation: Possibility that D N A methylation may cause an exposure level. The D N A methylation leads to exposure.

(A) Sources of prenatal exposures can include (clockwise from top left) dietary intake from food and supplements, pollution, psychosocial stress, and smoking and alcohol drinking behaviors. Exposures enter the pregnant person’s body typically through inhalation, ingestion, or dermal absorption. Once in the body, exposures are distributed and metabolized. They can interact with DNA in various tissues, including placental and fetal tissues. These are the major categories of exposure during pregnancy that were assessed for association with DNA methylation in this review article. (B) Common sources of information for exposure assessment include questionnaires, spatial sensors linked to participant residential history generally followed by exposure modeling, as well as blood- and urine-based exposure biomarkers. These methods are used by environmental health scientists to quantify individual levels of exposure to chemicals and to estimate the relevant time frame the exposure measure is applicable. (C) When we test for a relationship between an environmental exposure and DNA methylation, typically using multivariable regression methods, this is a test of association. Association tests do not indicate causation,4 because causal inference requires additional assumptions.5 In the case of environmental exposures and DNA methylation, a reverse causation scenario would mean the DNA methylation influences the exposure level. Although it is possible that for a given exposure level, differential methylation at genes essential for metabolism could influence measured concentrations of the contaminant (internal dose), methylation is unlikely to be causally related to being exposed. For this reason, reverse causation is less of a concern in studies of exogenous environmental exposures than in EWAS of disease or physiologic traits.6