In the mid-secretory phase endometrium of women with recurrent miscarriages, dense accumulations of uCD16(-) NK cells also occur, like those seen in first-trimester decidua of uncomplicated pregnancies.

This finding complicates understanding the exact roles of these NK cells at implantation sites. uCD16(-) NK cells are likely to be a mixture of indigenous endometrial NK cells and immigrant NK cells from the circulation. However, it is not yet known if NK cells from these two different origins display similar or unique characteristics. In this review, the potential underlying mechanisms for accumulation of uCD16(-) NK cells in uncomplicated pregnancies and in pathological pregnancies, especially recurrent miscarriages, are discussed.

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