Repeated Implantation failure (RIF) is the failure of the embryo to implant onto the side of the uterus wall following IVF treatment. Regularly, this happens at 6–7 days after conception and involves the embedding of the growing embryo into the mothers uterus and a connection being formed. A successful implantation can be determined by using an ultrasound to view the sac which the baby grows in, inside the uterus. However, the exact definition of RIF is debated. Recently the most commonly accepted definition is when a woman under 40 has gone through 3 unsuccessful cycles of IVF, when in each cycle 4 good quality eggs have been transferred. Repeated implantation failure should not be confused with recurrent IVF failure. Recurrent IVF failure is a much more broad term and includes all repeated failures to get pregnant from IVF. Repeated implantation failure specifically refers to those failures due to unsuccessful implanting to the uterus wall. An unsuccessful implantation can be down to either problems with the mother or with the embryo. It is essential that the mother and embryo are able to communicate with each other during all stages of pregnancy, and an absence of this communication can lead to an unsuccessful implantation and a further unsuccessful pregnancy. Contributing Maternal Factors During implantation, the embryo must cross the epithelial layer of the maternal endometrium before invading and implanting in the stroma layer. Maternal factors, including congenital uterine abnormalities, fibroids, endometrial polyps, intrauterine adhesions, adenomyosis, thrombophilia and endometriosis, can reduce the chances of implantation and result in RIF. Congenital Uterine Abnormalities Congenital uterine abnormalities are irregularities in the uterus which occur during the mothers foetal development. Hox Genes Two genes have been identified to assist in the development and receptivity of the uterus and endometrium, Hoxa10 and Hoxa11. Hoxa10 has been shown to change the upper uterine segment into oviduct-like structures, creating a smaller uterus that appears normal. Embryo transfer into the lower uterine segment does not allow for implantation, so the Hoxa10 gene has multiple effects throughout the uterus. Hoxa11 mutations alter the endometrial gland development and reduce the secretion of Leukaemia-Inhibitory factor (LIF) which is required for implantation. Fibroids Fibroids are benign tumours found in the smooth muscle of the uterus, they are often asymptomatic but can cause pelvic pain. They effect implantation rates by altering the shape and cytokine composition of the uterus. Removal of submucosal fibroids has shown