OBJECTIVE Maternal prenatal symptoms of depression and anxiety have been associated with altered neurodevelopmental outcomes in the child. These effects may be mediated in part by altered placental function, with increased fetal 5-hydroxytryptamine (5-HT) exposure being one possible mechanism. The current study aimed to determine whether maternal symptoms of depression or anxiety were associated with decreased placental expression of monoamine oxidase A (MAO A), the enzyme which metabolises 5-HT into 5-hydroxyindoleacetic acid. The localisation of MAO A in the placenta was also investigated. METHODS Pregnant women were recruited one day prior to elective caesarean and assessed using psychometric tests for symptoms of depression (Edinburgh Depression Scale) and anxiety (Spielberger State/Trait Index). Villous trophoblast tissue was extracted from each placenta and used for subsequent gene expression analysis (N=62). Localisation was studied using immunohistochemistry, with a specific polyclonal antibody. RESULTS Increasing symptoms of maternal depression were associated with a reduction in placental MAO A expression (r=-0.339, p=0.007, N=62). There was a trend for a similar correlation with symptoms of maternal trait anxiety, but not with state anxiety. MAO A was localised to the syncytiotrophoblast, the tissue between maternal and fetal blood. CONCLUSIONS These findings support the hypothesis that maternal mood is associated with altered placental function. A reduction in placental MAO A expression is consistent with a subsequent increase in fetal exposure to 5-HT.