['Cycle-dependent fluctuations in natural killer (NK) cell populations in endometrium and circulation may differ, contributing to unexplained infertility.', 'NK cell phenotypes were determined by flow cytometry in endometrial biopsies and matched blood samples.', 'While circulating and endometrial T cell populations remained constant throughout the menstrual cycle in fertile and infertile women, circulating NK cells in infertile women increased during the secretory phase. However, increased expression of CD94, CD158b (secretory phase), and CD158a (proliferative phase) by endometrial NK cells from infertile women was observed. These changes were not reflected in the circulation.'] Are changes in endometrial natural killer cell expression of CD94 , CD158a and CD158b associated with infertility?