The Role of Serotonin in Brain Development: From Molecular Pathways to Neurodevelopmental Risk.

Serotonin (5-hydroxytryptamine, 5-HT) is a multifaceted neuromodulator involved in processes from early embryogenesis to postnatal brain maturation. During pregnancy, 5-HT plays critical roles not only in maternal mood and behavior but also in placental function and fetal brain development. Beyond its classical role in mood regulation, 5-HT acts as a developmental signal influencing neuronal proliferation, migration, and synaptic organization within precisely timed critical windows. This narrative review provides an integrative framework linking molecular, cellular, and systems-level 5-HT mechanisms to neurodevelopmental outcomes, emphasizing how maternal, placental, and fetal sources of 5-HT dynamically interact with genetic, environmental, and pharmacological factors during gestation. By considering recent advances from developmental neurobiology, genetics, and psychopharmacology, we propose that disrupted 5-HTergic homeostasis represents a convergent mechanism underlying neurodevelopmental risk, particularly in autism spectrum disorder (ASD). We further highlight emerging evidence for sex-dependent 5-HTergic signaling and its role in differential susceptibility across offspring. These results collectively indicate the key role of 5-HT in neurodevelopment and highlight its promise as a biomarker and therapeutic target in neurodevelopmental disorders.