The implantation of the embryo and the proper development of the pregnancy depend on many factors, one of the most important of which is that the endometrium is perfectly ready. The endometrium is an organ that changes continuously during the menstrual cycle, under the influence of hormones, and is actually receptive for only a few days per month.
One of the quickest and easiest, but not the only way to know if the endometrium is in its receptive phase is to do an ultrasound scan and check its appearance, especially its thickness, because the endometrial tissue at the beginning of the menstrual cycle is very thin and throughout the cycle it grows day by day, up to a maximum that corresponds to the periovulatory period.
At that time the ideal thickness of the endometrium should be at least 7 mm to offer all the possibilities of a correct implantation. This measure has been established by rigorously studying thousands and thousands of treatments, observing that with endometriums above 7 mm the results were all very similar, while below 7 mm there were fewer pregnancies and more miscarriages, and the thinner the endometrium, the worse the results.
This does not mean that it is not possible to do an embryo transfer with endometrial thickness of less than 7 mm, in fact, pregnancies have been described with endometrial thickness of up to 4 mm, but we must be aware that the results will not be as good as if we manage to obtain a thicker endometrium.