Cyclical, hormone-driven molecular and morphological changes within the endometrium lead to a state of endometrial receptivity, when the embryo entering the uterine cavity is most likely to result in a successful pregnancy. Endometrial receptivity can be assessed directly by sampling the tissue, or indirectly by ultrasound. Direct sampling prior to embryo replacement in an in vitro fertilization cycle may decrease the chances of pregnancy; so indirect assessment is the only way to ascertain the optimum timing for embryo replacement. In a natural menstrual cycle, the endometrial thickness and volume increase, and the echo pattern changes form iso-echoic to hyperechoic in the luteal phase of the cycle. Endometrium in the assisted reproductive treatment cycles undergoes similar changes. Based on the currently available evidence, no single sonographic marker of endometrial receptivity exists. A combination of endometrial thickness in excess of 6 mm with a triple pattern appearance, an endometrial volume of over 2cm3, with present Doppler signal within the endo and sub-endometrium seem to be the most promising sonographic markers of endometrial receptivity. As the sonographic equipment continues to advance allowing for more detailed assessment of the endometrium, continuous search for sonographic endometrial receptivity markers is warranted.
Keywords: Assisted reproductive treatment, Doppler sonography, Endometrium, Ultrasound.