Optimal endometrial size is assessed by measuring endometrial thickness, which is the most commonly evaluated parameter. The imaging method of choice is transvaginal ultrasound.
The optimal endometrium prior to embryo transfer is the so-called trilaminar, triple line or "coffee bean" endometrium, typical of the advanced follicular phase. It is formed by two outer hyperechogenic lines that represent the junction between the basal endometrium and the myometrium; two hypoechoic bands that are the functional layer of the endometrium; and, finally, a central hyperechogenic line, the lumen of the cavity.
The literature suggests that the normal range of endometrial thickness would be 6 mm at the lower limit and 20 mm at the upper limit. Implantation is unlikely outside these ranges. A thickness between 7 and 9 mm at periovulatory time is considered ideal in the literature; however, reasonable pregnancy rates are also achieved with endometrial thicknesses of 6 mm.