Status of nucleolar channel systems in uterine secretions accurately reflects their prevalence-a marker for the window of implantation-in simultaneously obtained endometrial biopsies.

Objective: To assess whether nucleolar channel systems (NCSs) can be detected in exfoliated endometrial epithelial cells (EECs) of uterine secretions and whether such noninvasively determined NCS status is associated with significant NCS prevalence in simultaneously obtained endometrial biopsies.

Design: Prospective study (December 2015-February 2017).

Setting: University-affiliated and private fertility clinics.