Natural cycle/progesterone fortified protocol for endometrial preparation for frozen/thawed embryo transfer Study type: Descriptive pilot study of a novel protocol for endometrial preparation for frozen/thawed embryo transfer Introduction: In the process of in-vitro fertilization (IVF) cycles embryos are frequently cryopreserved for future attempts at conception. In order for these embryos to take root in the uterus, the endometrial lining must be made "receptive" to embryo implantation.