Much of our efforts in assisted reproductive technologies, namely IVF and IUI, is focused on cultivation of mature oocytes and generation of quality embryos. However, without a rich environment, these embryos cannot exist.
This soil is the endometrium, the lining of the uterus that is prepared monthly and then quickly shed like an afterthought, if implantation does not occur. The most notable observation of endometrial activity is the menstrual cycle. It may be thought of as the end of a cycle; probably because menstruation signals that a pregnancy did not occur. However, it actually represents a start as the body is preparing for another attempt at pregnancy.
Initially, the endometrium thickens as it is fueled by increasing estrogen hormone levels from the ovary. When ovulation occurs, it switches modes into a progesterone (Pro-gestational) dominant environment. This time is known as the luteal phase.
Within this phase exists a critical 24hr window that is often termed the “window of receptivitiy.” Frankly, it is the only time during the entire monthly cycle that the endometrium is competent to accept and to support the embryo.
The endometrium is incredibly dynamic during this time. The thin mucus-like coating that protected the endometrium dissolves, exposing the rich uterine environment that is teaming with receptors that are thought to attract the embryo and to be involved in its attachment and eventual implantation.
As the window comes to a close, the endometrium no longer is receptive to an embryo that has not implanted; and it begins the process anew. If implantation occurs, it promotes embryo growth and placental development as it plays host to its new and evolving tenant.