Figure 3.

Schematic outline of the experiment by Rudolph et al⁵⁶ testing the idea that menstruation is a secondary consequence of spontaneous decidualization

The experiment is conducted with the laboratory mouse, which is a species that under normal conditions is neither decidualizing nor menstruating. In this species, clitoral or vaginal stimulation during copulation leads to the maintenance of the corpus luteum even if no pregnancy ensued, leading to pseudopregnancy, as is the case by copulation with a vasectomized male. Furthermore, it is known that injection of a small droplet of oil into the uterine lumen causes decidualization. The experiment starts with mating a female to a vasectomized male to induce a pseudopregnancy. At the morning of the following day, the females are checked for a copulatory plug to verify that copulation has taken place. Then at day 4 after copulation, a small droplet of oil is injected into the uterus to induce decidualization. Day 4 is the normal day of implantation in mice. Then the mice are monitored for their level of progesterone and signs of vaginal bleeding. Progesterone starts to decrease after day 7, and bleeding ensues at about day 9. This experiment shows that differentiation of the endometrium (decidualization) is sufficient to cause menstruation-like symptoms in a species that normally does not menstruate.

Critchley. Menstruation: science and society. Am J Obstet Gynecol 2020.