Prenatal fentanyl and Δ9-tetrahydrocannabinol exposure disrupt placental function and fetal growth in a mouse model of multidrug use

Opioid and cannabis co-use during pregnancy is increasingly common, yet the placental mechanisms linking combined exposure to adverse fetal outcomes remain poorly defined. Using a controlled mouse model of gestational drug exposure, we tested how fentanyl, {Delta}9-tetrahydrocannabinol (THC), or their combination altered placental structure, immune signaling, and gene expression and how these changes related to fetal growth. Drug exposure produced fetal growth restriction and reduced placental efficiency, with the greatest impairment in the combined fentanyl+THC group. Placental alterations were detectable by mid-gestation, when fentanyl exposure increased nucleated red blood cells within the labyrinth, consistent with hypoxic stress. By term, placentas showed compartment-specific remodeling, with THC selectively expanding the labyrinth and combined fentanyl+THC exposure increasing decidual area. Labyrinth composition and proliferative activity were altered, ultrastructural analysis revealed disruption of the maternal-fetal exchange interface, and placental interleukin-10 and interferon-{beta} levels were reduced across exposure groups. Transcriptomic analyses identified suppression of innate immune and antiviral defense pathways together with treatment-specific stress responses, and integration of placental gene expression with fetal weight showed coordinated repression of vascular and developmental regulators and activation of hypoxia- and metabolic stress-associated genes. These findings identify the placenta as a key mediator of adverse fetal outcomes associated with prenatal polysubstance exposure.