**1. Introduction**

Pregnancy is an important event that leads to significant changes in maternal physiology. Successful pregnancy requires involvement of a series of processes commencing from fertilization to establishment of placental and maternal vascular connection with the fetus in correct order. Adequate placentation is one of the prerequisites for maintaining a normal healthy pregnancy. New insights into the placentation process involve migration, invasion, adherence, proliferation and differentiation of the placental principal cellular component, i.e., extravillous trophoblasts (EVTs), followed by their interaction with the pre-decasualized maternal uterine blood vessels, glands and lymphatics [1]. Placentation further evolves by digestion of the extracellular matrix where the EVTs tolerate surrounding maternal circulatory oxidative stress and the effects of soluble cytokines [1]. Nonetheless, the allogenic

EVTs also interact with maternal decidual immune cells to provide immune competence [2]. Any deviation in these events may lead to pathological pregnancies, i.e., preeclampsia (PE). The broad concept of PE pathophysiology includes defective trophoblast invasion and inadequate uterine spiral arterial remodeling in the first trimester that follows with reduced uteroplacental perfusion [3]. This subsequently leads to poorly perfused and stressed placental syncytiotrophoblasts that release a range of mediators causing endothelial dysfunction and PE clinical manifestations [3]. Moreover, such abnormal placentation leads to the secretion of abnormal levels of anti-angiogenic and inflammatory proteins that enter the systemic maternal circulation and impair maternal systemic vascular function, resulting in the clinical manifestations of PE. Since PE and its clinical symptoms rapidly abate after delivery (removal of the placenta), the placenta must play a central or initiating role in this pregnancy disorder.

Novel pharmacological interventions for the prevention of PE have not been developed for many years, as the complex pathophysiology, diversified clinical presentation of the disease and difficulties associated with conducting drug discovery research in pregnan<sup>t</sup> women have hampered their development. Low dose aspirin (LDA) is considered to be the most effective prophylactic therapy for reducing disease prevalence in women at high risk for developing early-onset PE. The use of LDA in pregnan<sup>t</sup> women is generally considered to be safe as it does not affect the pregnan<sup>t</sup> mothers and/or their unborn fetuses inadvertently. It has been suggested that the principal mechanism of action by which LDA exerts its effect is via the inhibition of thromboxane production that leads to the inhibition of platelet aggregation. Additionally, LDA has a direct positive effect on the villous trophoblasts [4]. However, recent evidence suggests that LDA prevents the development of PE by promoting trophoblast invasion and migration into the uterine arteries, interfering with cytokine production and stimulating the production of proangiogenic protein placental growth factor (PlGF); thereby, inhibiting apoptosis and premature uterine arterial remodeling [5].

Recent meta-analysis sugges<sup>t</sup> that LDA (≥100 mg/day) in early gestation (before 16 weeks) is beneficial in preventing common pregnancy complications; i.e., PE, fetal growth restriction, preterm birth [6–9], suggesting that aspirin may have effect on implantation and early placentation [10]. Low-dose aspirin has been utilized for many years to prevent PE [11–13]. A recent individual patient data meta-analysis observed that LDA can reduce the risk of PE development by 10% and small for gestational age (SGA) births by 24% [14] without posing a major safety risk to mothers or fetuses other than placental abruption in some cases [15]. Other studies reported that low dose aspirin is generally well tolerated within both preconception and early pregnancy periods [16].

In normal healthy pregnancy, placental syncytiotrophoblast release extracellular vesicles (EVs) including exosomes into the maternal bloodstream that contain some information (i.e., micro RNA, mRNA, proteins) to convey from the originating cells to their distant target cells such as maternal immune cells in order to adapt to the pregnancy associated physiological changes [17]. This EV release is further increased from the preeclamptic placenta due to oxidative stress, causing widespread systemic endothelial dysfunction, giving rise to maternal hypertension, feto-placental circulatory compromise and damaging various maternal organs [17]. Some recently published reports have suggested that LDA could influence platelet derived EV release; however, the effect of LDA on the regulation of placental EV release is not known. Therefore, in this review, we will discuss the potential mechanisms of action of aspirin in the context of PE prevention and the potential role of extracellular vesicles released from the placenta in this phenomenon (Figure 1).

**Figure 1.** Diagrammatic representation of preeclampsia (PE) development pathogenesis and mechanism of prevention by low dose aspirin (LDA). In PE, syncytiotrophoblast-derived extracellular vesicles (EVs), including exosomes, are released into the maternal circulation in increased amounts due to inadequate placental vascular remodeling. These EVs activate the vascular endothelial cells, leukocytes and platelets and cause dysfunction. LDA prevents the development of PE by reducingendothelial cell dysfunction. The proposed mechanism that was investigated; acetylsalicylic acid, the crude form of aspirin, modulates trophoblast derived exosome release and changes their proteomic and microRNA contents.
