**2.1 Endometrial receptivity**

Endometrial environment plays a crucial role in embryo implantation and early placental development. There is a certain period of endometrial maturation during which the trophectoderm of the blastocyst can attach to the endometrial epithelial cells and subsequently proceed to invade the endometrial stroma, which is called endometrial receptivity [14]. This complex process provides the embryo with the opportunity to normally attach, implant and develop.

There is a short period of time during the menstrual cycle, when the endometrial receptivity is optimal and embryo implantation is possible. This period is called "window of implantation" (WOI). Studies with donor embryos in humans have shown that this receptive period starts at day 6 post ovulation and continues 4–5 days (or 3–6 days) within the secretory phase (day 20–24 of the menstrual cycle) in most healthy women [15, 16]. Endometrium is unique in its ability to block embryos from implanting, except during this narrow window of receptivity, where endometrium undergoes morphological, cytoskeletal, biochemical, and genetic changes [17]. As shown in the mouse models (and is also true in the other species), WOI is regulated by ovarian steroid hormones. In the receptive endometrium, crucial hormones are progesterone and 17β-estradiol [18, 19]. In certain pathologic conditions, this window is narrowed or shifted, which disrupts normal implantation, leading to infertility or pregnancy loss [15, 16].
