Brosens, Jan2025-05-122025-05-122025: Joanne Muter1,2 †, Chow-Seng Kong1 †,Mireia Taus Nebot1 ,Maria Tryfonos1,3 , Pavle Vrljicak1 ‡, Paul J. Brighton1 , Danai B. Dimakou4 , Megan Vickers1 , Hiroyuki Yoshihara1 §, Sascha Ott1 , Bee K. Tan5 , Phillip R. Bennett5 , Siobhan Quenby1,2, Alex Richter4 , Hilde Van de Velde3 , Emma S. Lucas1 ¶, Thomas M. Rawlings1 , and Jan J. Brosens1,2*http://hdl.handle.net/20.500.14200/7709In every menstrual cycle, progesterone acting on estrogen-primed endometrium elicits an inflammatory decidual reaction, rendering it poised for embryo implantation and transformation into the decidua of pregnancy. Here, we show that the sequential functions of the decidual reaction - implantation and decidualization - pivot on the time-sensitive loss of progesterone-resistant DIO2+ stromal cells that form a specialized implantation niche and reciprocal expansion of progesterone-dependent PLA2G2A+ pre-decidual cells. Simultaneously, uterine natural killer (uNK) cell proliferation results in the accumulation of immunotolerant subsets. Examination of endometrial biopsies from 924 women revealed that the recurrence risk of miscarriage closely aligns with the incidence of a weakened or stalled decidual reaction, more so than poor uNK cell expansion. Analysis of paired biopsies obtained in different cycles and modeling in assembloids intimated that prior miscarriages disrupt intercycle endometrial homeostasis and calibration of the decidual reaction. Our findings show that erosion of the decidual reaction following a miscarriage drives the recurrence risk irrespective of maternal age.enArticle