NK cells accumulate in adult murine and human uteri during decidualization induced physiologically, pathologically, or experimentally. Adoptive transfer studies indicate that uterine NK (uNK) cells arise from circulating progenitors. However, virgin uteri contain few circulating NK1.1+CD49a2 conventional NK cells, whereas NK1.1+CD49a+ tissue-resident NK (trNK) cells are abundant. In this study, we employed a novel, immune-competent NK cell-specific reporter mouse to track accumulation of uNK cells during unmanipulated pregnancies. We identified conventional NK and trNK cells accumulating in both decidua basalis and myometrium. Only trNK cells showed evidence of proliferation. In parabiosis studies using experimentally induced deciduomata, the accumulated uNK cells were proliferating trNK cells; migrating NK cells made no contribution. Together, these data suggest proliferating trNK cells are the source of uNK cells during endometrial decidualization.