Recurrent implantation failure (RIF) occurs after women undergo in vitro fertilization and have multiple embryos transferred but no resulting pregnancies. RIF is a very challenging clinical entity for the reproductive physician and the patient. In fact, there is not even an agreed upon definition in the medical community. Many potential causes have been investigated over the past several years but no clear answer has emerged. Interest has recently turned to the endometrium, or the lining of the uterus. Studies have shown that the genes that are turned on in the endometrium vary based on how long this tissue has been exposed to progesterone, an important hormone that prepares the uterine lining for implantation of the growing embryo. In some women, it seems that they require longer periods of progesterone exposure to reach the same state of readiness. We hypothesize that an even larger proportion of women in RIF population will require longer treatments with progesterone.

In this study, we will randomize women with RIF who are about to undergo a frozen embryo transfer to one of two groups. The first group will have their embryo transfer done on the standard sixth day of progesterone treatment. The other group will have their transfer done on the seventh day of progesterone. We will be comparing the clinical pregnancy rate of the two groups as well as the live birth rate and miscarriage rate. We expect that extending the progesterone treatment by one day will increase the pregnancy rate of women with RIF.

Inclusion Criteria:

• Patient at Mount Sinai Fertility who wants to undertake a frozen embryo transfer
• History of recurrent implantation failure, defined as no pregnancy after three or more embryo transfers of four or more good quality blastocysts

Exclusion Criteria:

• Previous testing with Endometrial Receptivity Assay (ERA)
• Unable understand/communicate in English