(A) Experimental protocols. The artificial menstrual cycle. Sequential treatment with E2 alone (Proliferative Phase) and then E2 + P (secretory phase) primes the endometrium for menstruation during the Luteal Follicular Transition (LFT) after P implant removal. (B) Assessment of P add back on menses. P implants were removed at the end of the cycle. P implants were then replaced in the animals at 12, 24, 30 36, 40, 48, 60, or 72 hours (h) after initial P withdrawal. Control animals (n = 1/time point) received blank Silastic implants. (C) Assessment of P add back on MMPs. P was withdrawn as described above, and then either replaced at 12 and 24 hours during the critical period (Group 1), or at 48 hours after the critical period (Group 2). Tissues were collected at 48 h for group 1 and 72 hours for group 2. Control animals did not have P implants replaces and tissues were collected at either 48 or 72 h after P withdrawal.