Through in vitro fertilization (IVF), the investigators have seen that the correct thickness of endometrium is a marker of successful implantation and ongoing pregnancy, although the reason for this is not entirely clear. In order to better understand the processes that may occur in the endometrium, the investigators are conducting a study which evaluates biochemical markers of those patients who have shown a failure to proliferate during previous synthetic IVF frozen cycles and biochemical markers of control patients who have no known endometrial pathology.