The purpose of this study is to determine whether the chances of becoming pregnant are better when day the single best day 5 embryo (blastocyst) resulting from an in vitro fertilization (IVF) cycle is transferred into the uterus immediately, or after freezing the embryo and transferring it into the uterus in a subsequent cycle, separate from the ovarian stimulation used in the IVF cycle. The investigators hypothesize that in good-prognosis patients, vitrified-warmed elective single embryo transfer will result in higher implantation, clinical and on-going pregnancy and live birth rates than fresh elective single embryo transfer at the blastocyst stage.