Most authors coincide that an endometrial thickness lesser than 8 mm has a very low pregnancy rate. To determine if the measurement of endometrial thickness by vaginal ultrasound has a prognostic value over the pregnancy rate in patients with endometrial preparation for an embryo transfer in cycles of frozen embryos and ovum donation. Retrospective and comparative. A total of 100 cycles were analyzed in the group of frozen embryo transfer; and 71 cycles in the group of ovum donation, we subdivided these two groups into two more groups, one with an endometrial thickness of 8 mm or less and the other with more than 8 mm. There is a significant difference in the group of frozen embryo transfer regarding the age of the patients (p = 0.009). The pregnancy rate in the group of frozen embryo transfer with an endometrial thickness of 8 mm or less was 14.7% and of 37% in the group with more than 8 mm, with a significant difference (p < 0.01). When we analyzed the cycles of ovum donation, we did not found a significant difference, and we found a pregnancy rate of 47% in the group with endometrial thickness of 8 mm or less, and 40% in the other group. In the present study we found that the measurement of the endometrial thickness of more than 8 mm in the patient undergoing a frozen embryo transfer has a predictor value.