PROJECT SUMMARY Pregnancy is a unique state in which maternal gestational weight gain (GWG) is physiologically adaptive and necessary. Excessive gestational weight gain (EGWG) is weight gain exceeding guidelines and contributes to long term maternal metabolic dysfunction. While many diet- and activity-based intervention trials have been performed to limit EGWG, the data are inconsistent regarding successfully preventing EGWG in pregnancy. Potentially, the impediments faced in successfully mitigating EGWG arise from our lack of clarity about mechanisms underlying physiologic GWG. Therefore, the concept that diet and physical activity fully account for quantitative GWG variation in pregnancy is far too simplistic. Weight regulation is a complex process involving interactions among multiple neurobiological and endocrine pathways, but the central control predominantly occurs in the mediobasal hypothalamus (MBH), specifically the arcuate nucleus. Recent studies in nonpregnant humans demonstrate that gliosis (a cellular inflammatory response) in the MBH is associated with obesity. Studies in rodents exposed to a high-fat diet show that MBH gliosis precedes weight gain and that these glial inflammatory responses are both necessary and sufficient for weight gain. In humans, increased T2 signal on T2-weighted brain MRI is a radiologic marker of gliosis. Increased T2 signal in the MBH associates with obesity, insulin resistance and increased visceral adipose tissue (VAT), independent of body mass index (BMI). VAT, fat surrounding intra-abdominal solid organs, produces more pro-inflammatory cytokines than subcutaneous fat and is associated with increased risk for metabolic disease. We, and others, have shown associations between increased VAT and gestational metabolic disease. Therefore, based on rodent literature and findings in non- pregnant humans, it seems plausible that MBH gliosis could associate with pathological GWG or affect preferential deposition of VAT during pregnancy. Foundational data from this R21 proposal will provide the first information regarding a neural mechanism underlying dysregulated or pathological weight gain in pregnancy.