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Problem Vaginal bleeding during early pregnancy is estimated to occur in 20% of all pregnancies and it is often difficult to predict who will ultimately miscarry. The role of immune cells in early pregnancy loss is poorly understood. Method of Study In this prospective cohort study, 28 pregnant women presenting with first‐trimester vaginal bleeding donated vaginal blood, peripheral venous blood, and saliva during their initial emergency room visit, and at a follow‐up. The composition, frequency, and phenotype of immune cells in the vaginal blood were determined using flow cytometry. The proteome of serum and saliva was analyzed with OLINK proximity extension assay and correlated to vaginal immune cell phenotype and outcome of pregnancy. The course and outcome of pregnancies were followed and recorded. Results Vaginal blood contained all main immune cell lineages including B cells, NK cells, T cells, and monocytes/macrophages. Notably, vaginal blood immune cells expressed tissue residency markers including CD49a. Women who subsequently miscarried had a higher frequency of vaginal blood CD49a+ NK cells compared to those who did not miscarry, and this correlated with serum levels of granzyme A and H, as well as CSF1, CAIX, and TWEAK. Women in the miscarriage group also had a higher frequency of peripheral blood T cells expressing CD49a. Conclusions Our study provides novel insight into human reproductive immunology in relation to miscarriage. Tissue‐resident NK cells in vaginal blood alone or in combination with serological biomarkers hold potential as prognostic factors in the prediction of pregnancy outcome in women with early pregnancy bleedings.

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