Therefore, we examined the effects of high glucose (25 mM) and insulin (10 nM)—two hallmarks of maternal obesity and GDM—on mRNA expression of key regulators of serotonin homeostasis, including serotonin transporter (SERT), tryptophan hydroxylase 1 (TPH1), and monoamine oxidase A (MAOA), in the first-trimester trophoblast cell line ACH-3P, focusing on oxygen levels characteristic of early human placental development. The gene discussed is MAOA; the disease is gestational diabetes.