The purpose of this ongoing study is to look at whether Endometrin® (vaginal micronized progesterone tablets) supplemented by intramuscular injection of progesterone in oil (PIO) work as well as PIO alone for women undergoing transfer of frozen-thawed blastocyst(s). Another goal of the study is to determine whether patients prefer Endometrin or PIO. Endometrin® has been approved by the United States Food and Drug Administration, or FDA, "to support embryo implantation and early pregnancy by supplementation of corpus luteal function as part of an Assisted Reproductive Technology (ART) treatment program for infertile women." (FDA New Drug Approval Letter, Endometrin®, 2007). The use of Endometrin® (vaginal micronized progesterone tablets) in this study is investigational. An investigational use is one that is not approved by the U.S. Food and Drug Administration (FDA).