Introduction: Peripartum depression (PPD) is an episode of depression occurring during pregnancy or up to one year postpartum. Affecting around 19% of women, it is among the most common psychological problems during the peripartum period. PPD is a complex disorder shaped by various biological and psychosocial risk factors, and it is associated with numerous adverse outcomes for mothers, children, and overall family functioning. Studies of the biological underpinnings of depression in the general population have highlighted the role of serotonin, a neurotransmitter involved in regulating mood and numerous physiological functions. A novel and promising area of depression research is the field of epigenetics, particularly studies of DNA methylation – a potentially reversible and heritable epigenetic modification of DNA that regulates gene expression and is influenced by various environmental factors. Aberrant methylation patterns are associated with different illnesses, including depression. Epigenetic studies could potentially delineate the underlying biological mechanisms of diseases and improve diagnosis and personalised treatment. However, epigenetic mechanisms, such as the methylation of the serotonin transporter gene (SLC6A4), have been very scarcely explored in relation to PPD. Additionally, studies on the genetic basis of PPD, such as those examining the serotonin-transporter-linked polymorphic region (5- HTTLPR) and its rs25531 variant, are both rare and inconclusive. Furthermore, comprehensive models of PPD that integrate both biological and psychosocial risk factors and their interrelations are lacking. As a result, the aetiology of PPD remains largely unknown, with the role of the serotonin system particularly unclear. Objectives: To address the prominent knowledge gaps in the literature, the objective of this study was: 1) to examine the biological factors (whole blood serotonin concentration, polymorphism and methylation of the SLC6A4 gene) and psychosocial factors (stress – quantity of stressors per each trimester and perceived stress, anxiety – general and pregnancy-specific anxiety, and social support – from family, friends, and partner) in predicting depression during pregnancy and postpartum; and 2) to examine the interrelationship between (epi)genetic factors and stress for depression during pregnancy and postpartum, as the a) moderation of the 5- HTTLPR/rs25531 polymorphism, and b) mediation of the SLC6A4 methylation, of the relationship between stress and PPD. Methods: A longitudinal two-wave study was conducted on 202 pregnant women recruited a few days before a planned cesarean section at the University Hospital Centre Zagreb. Blood samples were collected, and participants completed questionnaires containing general demographic questions and psychosocial instruments. The questionnaires comprised the General Data Questionnaire, the Edinburgh Postnatal Depression Scale (EPDS), Depression, Anxiety and Stress Scales (subscales Anxiety and Stress) (DASS-21), Pregnancy Concerns Scale (PCS), Social Readjustment Rating Scale (SRRS) for each trimester, Social Support Appraisals Scale (subscales Family and Friends) (SS-A), and Perceived Support from Partner Scale (PSPS). In addition, demographic, anthropometric, and clinical information were extracted from medical records. Women with multiple pregnancies, diagnoses of type I or II diabetes, fetal anomalies, and preterm delivery were excluded. The second measurement was around three months postpartum via online questionnaires that included all the previous psychosocial questionnaires except the SRRS and PCS. Serotonin concentration was expressed as mass per whole blood volume (ng/mL), as well as mass per platelet count (ng/109 platelets) and platelet volume (ng/µL). Genotypes of the triallelic 5-HTTLPR/rs25531 polymorphism were categorised into low-expressing (S and LG allele carriers, i.e., genotypes S/S, S/LA, S/LG, LG/LA, LG/LG) and high-expressing (homozygous LA/LA genotype) groups. All analyses were performed with biallelic 5-HTTLPR and triallelic 5-HTTLPR/rs25531 separately. DNA methylation of the SLC6A4 was expressed as the average methylation level across 15 CpG loci. Results: The results showed that blood serotonin levels, genetic polymorphism, and methylation of the SLC6A4 were not associated with depression symptoms in late pregnancy or postpartum. Among psychosocial factors measured in pregnancy, higher perceived stress and pregnancy-specific anxiety were significantly associated with antenatal depression symptoms, while higher depression symptoms in pregnancy, higher perceived stress and lower support from friends and partner in postpartum period significantly explained postpartum depression symptoms. Among the biopsychosocial factors measured in pregnancy, only antenatal depression symptoms predicted postpartum depression symptoms. Additionally, a significant interaction between the 5-HTTLPR /rs25531 polymorphism and stress was found in predicting PPD at both time points. Specifically, higher number of stressors in the second trimester was associated with elevated depression symptoms both during pregnancy and postpartum, with this effect being more pronounced in LA/LA genotype carriers compared to S and LG allele carriers. Conversely, higher perceived stress in pregnancy was associated with higher antenatal depression symptoms, with a more pronounced effect observed among S and LG allele carriers compared to LA/LA genotype carriers. No mediating role of SLC6A4 methylation in the relationship between antenatal stress and postpartum depression symptoms was observed. Conclusion: The findings underscore the complex and multifaceted nature of PPD, which is shaped by distinct risk factors in late pregnancy and postpartum. Psychosocial adversity, particularly higher stress and pregnancy-specific anxiety were related to antenatal depression symptoms, while antenatal depression, higher perceived stress and lower support from partner and friends in the postpartum were related to postpartum depression symptoms. Stress had a central role in PPD, demonstrating a consistent direct association and an interaction with 5- HTTLPR/rs25531 genetic polymorphism. This genetic variability modulated depressogenic response to stress, supporting the gene-environment interaction hypothesis and the fundamental postulates of the biopsychosocial model of illness and health. The results highlight the intricate interplay between biological and psychosocial factors, thereby reinforcing the need for adopting a biopsychosocial approach to PPD in future research and clinical practice. Comprehensive and timely assessments of PPD across the peripartum period can foster targeted prevention, early recognition, and personalised treatment to mitigate the substantial burden of this prevalent and severe disorder.