The placental tryptophan pathway across gestation: implications for pregnancy outcomes

BACKGROUND: Tryptophan metabolism within the placenta generates bioactive metabolites, including serotonin (5-hydroxytryptamine; 5-HT), melatonin, and kynurenine derivatives, that regulate immune tolerance, vascular function, oxidative balance, and fetal neurodevelopment. Increasing evidence indicates that placental handling of tryptophan is dynamically regulated across gestation and is highly sensitive to maternal environmental and metabolic cues. OBJECTIVE AND RATIONALE: The aim of this review is to examine placental tryptophan metabolism across gestation, with a focus on the 5-HT, melatonin, and kynurenine pathways. We address how these pathways are regulated during normal pregnancy and how maternal factors, including inflammation, hypoxia, oxidative stress, and cardiometabolic dysfunction, influence placental tryptophan handling in pregnancy complications such as early pregnancy loss, preeclampsia, fetal growth restriction, and preterm birth. SEARCH METHODS: PubMed was searched using predefined terms related to placental tryptophan metabolism, 5-HT, melatonin, kynurenine, fetal programming, neurodevelopment, and pregnancy complications. Only full-text, peer-reviewed articles published in English were included. Abstracts and conference proceedings were excluded due to their limited data reliability. OUTCOMES: Placental tryptophan metabolism shows clear gestational stage-dependent regulation, and early pregnancy emerges as a formative period when pathway activity and metabolite balance are first established. From early pregnancy, maternal-decidual kynurenine pathway activity and placental 5-HT synthesis intersect with immune tolerance, vascular adaptation, and neurodevelopmental signaling. Across gestation, maternal inflammation, hypoxia, oxidative stress, and cardiometabolic disturbance can redirect the tryptophan flux and shift the balance between 5-HT/melatonin and downstream kynurenine metabolites. Evidence across pregnancy complications links early pathway disruption to pregnancy loss and supports the view that early metabolic perturbations contribute to vulnerability for later placental dysfunction, including preeclampsia, fetal growth restriction, and preterm birth. WIDER IMPLICATIONS: Placental tryptophan metabolism changes across gestation, making early pregnancy a critical window when pathway balance and fetal exposure to neuroactive metabolites are first set. Maternal inflammation, metabolic status, nutrition, and drug exposures may alter this balance, with the placenta acting as the key interface that transmits maternal signals to the fetus and shapes neurodevelopmental trajectories. To define the clinical relevance of altered tryptophan catabolism, longitudinal human studies are needed to link placental phenotypes with pregnancy outcomes and postnatal neurodevelopment. These should be complemented by mechanistic models that resolve regulation in early gestation. REGISTRATION NUMBER: n/a.