Spontaneous abortion (SA) is a common adverse pregnancy outcome in obstetrics and gynecology. It is characterized by various clinical manifestations such as cessation of embryonic development, embryonic or fetal death, and expulsion of the embryo and its appendages. It is estimated that 10–20 % of clinical pregnancies end in SA (Griebel et al., 2005; Rai and Regan, 2006; Zheng et al., 2017), and the incidence of SA may be close to 30 % considering the occurrence of early pregnancy loss (Alves and Rapp, 2023). Notably, international disparities persist in the definition of SA. The Royal College of Obstetricians and Gynecologists (RCOG) (Gynaecologists, 2011), and the European Society for Human Reproduction and Embryology (ESHRE) propose the criterion of pregnancy loss before 24 weeks of gestation as the threshold for SA (Bender Atik et al., 2018). Conversely, SA is defined in China as pregnancy termination at <28 weeks of gestation and fetal weight of <1000 g. The etiology of SA remains enigmatic and intricate, potentially encompassing genetics, autoimmunity, adverse environmental exposures, lifestyle influences, and past medical histories (Griebel et al., 2005; Grippo et al., 2018; Wu et al., 2016; Zhu et al., 2022). The growing maternal exposure to harmful factors has, to a certain extent, contributed to an increasing prevalence of SA. That is gradually becoming an important event affecting the health of women of reproductive age.

The health risks posed by SA to women can be divided into physical and psychological. Physiologically, SA may lead to infection, uterine adhesions, bleeding, and even maternal death (Apgar and Churgay, 1993; Bulletti et al., 1996; Friedler et al., 1993; Hooker et al., 2014; Wang et al., 2021). In recent years, with the improvement in living standards, there has been an increasing concern regarding the psychological impact of this condition. A meta-study confirmed that mental disorders are globally prevalent, affecting people in all regions of the world, and that the prevalence of mood disorders and anxiety disorders is higher in women than in men (Steel et al., 2014). Bipolar disorder and major depression (BDMD), a complex psychiatric disorder characterized by recurring episodes of mania, hypomania, major depression, and mixed mood (Cheng et al., 2023). Bipolar I disorder is typified by one or more major depressive episodes accompanied by one or more manic or mixed episodes. Bipolar II disorder is characterized by one or more major depressive episodes alongside at least one hypomanic episode (Angst et al., 2003). Although the exact pathogenesis of BDMD remains unclear, the processes involve disruptions in neuronal-glial plasticity, monoaminergic signaling, inflammatory homeostasis, cellular metabolic pathways, and mitochondrial function (McIntyre et al., 2020).

Whether preexisting SAs cause BDMD is a question that remains open. The current phase of research suggests that SA is associated with a heightened likelihood of experiencing anxiety, depression, and post-traumatic stress disorder (Farren et al., 2020). Miscarriage is associated with perinatal suicidal ideation, anxiety, and other mood disorders (Bright et al., 2022). However, some studies have claimed that the incidence of mania or affective psychosis after a full-term delivery is significantly higher than that of SA (Di Florio et al., 2015). Women with prior SA were not at increased risk for depressive reactions or other psychiatric symptoms in response to a recurrence of SA (Toffol et al., 2013). There is still a lack of studies reporting SA and BDMD as a severe mixed affective disorder. Few studies have focused on whether the risk of BDMD is increased after SA. Moreover, previous investigations have neglected to explore the potential dose-response relationship between the number of SAs and the risk of developing BDMD among women. Therefore, the purpose of this study was to examine the association between the number of SAs and the incidence of BDMD. We hypothesized that there might be some association between SA and BDMD. By doing so, this study aims to provide a foundation for implementing comprehensive preventive measures to address BDMD among women who have undergone SA.