Endometrial Immune Profile in Recurrent Implantation Failure

The goal of this observational study is to learn whether differences in the immune cells of the womb lining (endometrium) help explain why some women do not become pregnant despite several IVF attempts using good-quality embryos, a condition known as recurrent implantation failure (RIF). The study compares women with RIF to women who have had a natural, healthy pregnancy. The main question it aims to answer is: Do immune cells in the endometrium, including uterine natural killer cells, macrophages, B cells, and T cells, differ in number or function between women with RIF and fertile controls? Researchers will compare endometrial tissue and blood samples from the RIF group to samples from the fertile control group to see whether specific immune cell patterns are linked to implantation failure, and whether these differences could point to future treatment targets. Participants will: * Attend a consent appointment to discuss the study * Have a transvaginal ultrasound scan, if one has not already been done in the past 6 months * Confirm the timing of their cycle using a home ovulation test, an ultrasound scan, and a blood test * Have an endometrial tissue sample and a blood sample taken at the point in their cycle when the womb lining would normally be ready to accept an embryo