The increasing success of in vitro fertilization (IVF) has come mainly from advances in the way doctors grow and select embryos. When transferred into a woman's womb, however, only a minority of these embryos implant in the lining of the uterus, also known as the endometrium. “The reason,” says Steven L. Young, professor of obstetrics and gynecology at the University of North Carolina at Chapel Hill, “lies primarily with an inability to evaluate if the endometrium is ready for the embryo.” If it is not, embryos will not implant, much like good seed will not grow in bad soil.