According to an article in the July 2007 Fertility and Sterility journal, changes in the thickness of the uterus lining during in vitro fertilization predict the likelihood of pregnancy.

Those patients who show a quick endometrial response have a much better pregnancy outcome than those who have a slower response. The women that had a thickening of the endometrium more quickly were the ones that became pregnant. It was noted that the thickness of the endometrium while undergoing the procedure had no effect on pregnancy rate, but rather the speed of change seemed to predict the success of the procedure.