Placental adaptation to maternal overnutrition as an evolutionary trade-off.

The mammalian placenta is genetically identical to the fetus, exposing it to unique selective pressures. In humans, placental evolution has been explored through the lens of nutritional scarcity, with selective pressures assumed to diminish as resources increase. Here, we challenge this assumption, arguing that maternal overnutrition constitutes a distinct and underappreciated selective pressure. Maternal inability to maintain fuel homeostasis exposes the fetus to nutrient overload, elevating fetal insulin and oxygen demand. Selection has favored placental adaptive responses to the resulting oxygen deficit, with insulin as the mechanistic core. However, the role of fetal hyperinsulinemia in protecting brain oxygen supply comes at the cost of excess fetal adiposity. This trade-off carries downstream health risks, including obstetric risks, preterm birth, and intergenerational metabolic disease.