The serotonin receptor 5-HT2A (encoded by HTR2A) is an important regulator of fetal brain development and adult cognitive function. Environmental signals that induce epigenetic changes of serotonin response genes, including HTR2A, have been implicated in adverse mental health outcomes. The objective of this perspective article is to address the medical implications of HTR2A epigenetic regulation, which has been associated with both infant neurobehavioral outcomes and adult mental health. Ongoing research has identified a region of the HTR2A promoter that has been associated with a number of medical outcomes in adults and infants, including bipolar disorder, schizophrenia, chronic fatigue syndrome, borderline personality disorder, suicidality, and neurobehavioral outcomes. Epigenetic regulation of HTR2A has been studied in several different types of tissues, including the placenta. The placenta is an important source of serotonin during fetal neurodevelopment, and placental epigenetic variation of HTR2A has been associated with infant neurobehavioral outcomes, which may represent the basis of adult mental health disorders. Further analysis is needed to identify intrinsic and extrinsic factors that modulate HTR2A methylation, and the mechanism by which this epigenetic variation influences fetal growth and leads to altered brain development, manifesting in psychiatric disorders.