There are many commercial tests for blood immune cell tests looking at NK cell numbers, cell cytotoxicity or associated cytokine levels. Despite the lack of evidence, patients with recurrent pregnancy loss and implantation failure are often advised to have these tests and are subsequently offered immunotherapies, not backed by trial evidence to treat apparently abnormal results. Blood NK cells are innate immune cells and a first line of host defence against pathogens and tumour cells. Uterine NK cells are phenotypically different to blood NK cells and function when activated to promote normal placentation when pregnancy occurs. Despite a lack of biological plausibility, meta-analyses have been suggestive of altered blood NK cells in subfertility and recurrent pregnancy loss. However they have not shown that blood NK cells have an impact on pregnancy outcome. It is imperative that we offer patients tests and treatment based on robust evidence rather than poor science.