Uterine receptivity is defined as the capacity of the uterus that allows the embryo to implant. In IVF treatments this mechanism has been studied and is known. It implies certain molecular and structural changes that will allow – or not – the implantation of the developing embryo. There are certain days that are best for the embryo implantation to take place, known as implantation window, which takes place 5-6 days after ovulation.
The definition of the optimal moment for the embryo transfer has been tried in several studies through biopsy and immunohistochemistry. Also electron microscopy allows seeing the elongations of the surfaces of the endometrial cells named pinopods, which have been related with the best moment for the implantation to take place.
Nowadays, the study with flow cytometry and microarrays of the sinthesised proteins (integrins), as well as the expressed genes by the endometrial cells in a certain moment are the biomarkers that allow the expert to estimate the best moment for implantation. In a near future they may become crucial studies.
At the same time, the professionals of assisted reproduction, through the study of the uterus and endometrium through ultrasounds, search for sonographic and Doppler signs that point the highest moment of receptivity of the uterus.
Abdominal ultrasounds, before and after the vaginal 2D and 3D ultrasound, give some parameters of uterine receptivity that will indicate the real possibilities that the implantation takes place.
A thickness of the endometrium superior to 8mm, the apparition of intraendometrial vessels the day of the embryo transference and endometrial volumes higher than 2,5 ml are the current published parameters that indicate a good uterine receptivity.
Researches about this subject show that approximately 25% of women that turn to assisted reproduction techniques have low uterine receptivity, characterised by a hysterometry inferior to 6.5cm, endometrial thickness under 8mm and DIO inferior to 23mm. Furthermore, they found a link between this group of women and those diagnosed of unknown sterility or polycystic ovary syndrome.
If the pregnancy rate of these women is valued according to the endometrial thickness, it can be observed that gravidity takes place mainly with endometrial thickness of 9-12mm, being significantly lower with thickness inferior to 8mm or over 14mm.
To sum up, ultrasound, as well as biomarkers studies, gives us information about the uterine receptivity and the optimal conditions for implantation to take place and has the advantages of being cheaper, harmless and that it can be repeated.