Historically, we have been limited the means of assay endometrial receptivity. It is a dynamic organ that needs to be evaluated as such. Embryo implantation occurs when the endometrial receptivity is precisely well orchestrated. When the window of receptivity does not meet optimal implantation timing of the embryo, pregnancy will not occur.
Endometrial biopsies where a random representation of tissue is taken, has been one of the mainstays of assessment. Biopsy results were aimed at looking for pathology (such as inflammation and infection) as well as dating of the tissue to correlate it to what is happening at the level of the ovary.
The advent and use of transvaginal ultrasound has replaced much of what is assayed with biopsy regarding endometrial dating. Such ultrasound is used for monitoring ovarian stimulation during an ART cycle (IUI or IVF). As a part of these ultrasounds, basic elements of endometrial growth and development are followed.
This modicum plays a key role in timing for frozen embryo transfer cycles and donor egg programs, in particular. From the body of evidence accrued, we know estrogen and progesterone are the only two hormones necessary to provide for an adequate endometrium. The length of time of estrogen exposure does not seem to be as important (5-30days) as is the development and resultant thickness of the endometrium prior to progesterone exposure.
The consensus seems to be that an endometrial thickness of > 7mm seems to be adequate. Thinner endometrial linings (<6mm) are associated with earlier delivery rates.
There are many speculations regarding what makes an ideal thickness for the endometrial lining. It does not appear to be just a matter of thickness or cushioning. (As an example, a plush mattress is always preferred to a sleeping bag.) There are probably many factors contributing to increased chances of successful embryo implantation.