**1. Introduction**

Uterine function is key for a suitable reproductive performance and fertility. Endocrine and immune response play important roles for keeping the hormonal levels and the fetal-maternal interface. Hormones such as estrogen and progesterone are released from the ovaries; estrogen release is triggered by the hypothalamicpituitary axis, and progesterone is secreted from the corpus luteum (CL) after ovulation. Estrogen is essential for uterine growth and cell proliferation and progesterone for endometrial receptivity and successful establishment of pregnancy [1, 2]. Estrogen and progesterone are also considered as regulators of innate immunity and inflammation in the endometrium [3]. Endometrial immune homeostasis plays an important role in the success of implantation and pregnancy, with complex interactions between the innate and adaptive immune system, through cells such as natural killer, antigen-presenting cells (macrophages and dendritic cells), and subtypes of T cells [4].

Prostaglandins (PGs), also known as prostanoids, are bioactive lipids with an important function as regulators of reproductive processes, including ovulation, fertilization, and implantation. PGs are synthesized from arachidonic acid by different cells, and five types of PGs have been described, with specific roles and mechanism in the female reproductive system [5, 6]. Prostaglandin F2α (PGF2α) has a luteolytic effect, whereas prostaglandin E2 (PGE2) is central in ovulation, fertilization, embryo development, and implantation [7, 8]. In addition, PGE2 plays important roles in inflammatory processes, being increased at first phases of inflammation [9].

Inflammation is a complex process with two differentiated steps or conditions: acute and chronic inflammation. A number of lipid mediators act as pro-inflammatory (i.e., leukotriene and prostaglandins) or anti-inflammatory and pro-resolving (lipoxins, resolvins, maresins, and protectins) mediators. Lipid mediators derived from polyunsaturated fatty acids (PUFA) have potent anti-inflammatory effect and promote the resolution of inflammation, through specialized pro-resolving lipid mediators (SPM) [10]. Omega-3 fatty acids are a type of PUFA with known beneficial effect, which, in addition to its pro-resolving mechanism, have shown two additional anti-inflammatory mechanisms: activation of the free fatty acid (FFA)-4 receptor and inflammasome inhibition. Recent evidences have suggested an anti-inflammatory effect of docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) in the endometrium. Furthermore, FFA4 receptor was detected in the human, mouse, and bovine uterus [11, 12]. The following sections describe the effect and potential mechanisms of omega-3 fatty acids in the immune cells and endometrium and perspectives of these fatty acids in health and disease.
