The major leukocyte population in the decidua during the first trimester of pregnancy consists of NK cells that express receptors capable of recognizing MHC class I molecules expressed by placental trophoblast. These include members of the killer immunoglobulin-like receptor (KIR) family, the two-domain KIR (KIR2D), which recognize HLA-C. Interactions between decidual NK (dNK) cell KIR2D and placental HLA-C contribute to the success of pregnancy and dNK cells express KIR2D at higher frequency than peripheral NK (pNK) cells. Thus, they are biased toward recognizing HLA-C. In order to investigate when this unusual KIR repertoire appears, we compared the phenotype of NK cells isolated from non-pregnant (endometrium) and pregnant (decidua) human uterine mucosa. Endometrial NK (eNK) cells did not express KIR2D at a higher level than matched pNK cells, so the bias toward HLA-C recognition occurs as a response to pregnancy. Furthermore, HLA-C expression was upregulated on uterine stromal cells as the mucosa transformed from endometrium to decidua at the onset of pregnancy. As uterine NK (uNK) cells can mature from NK precursors and acquire KIR expression in utero, the pregnancy-specific bias of uNK cells toward HLA-C recognition could arise as developing uNK cells interact with uterine stromal cells, which express higher levels of HLA-C during pregnancy.