A new study in mice reveals how progesterone inhibits the growth-promoting actions of estrogen in the uterus during pregnancy (Li Q et al. Science. 2011;331:912-916).
Researchers found that progesterone induces expression of the transcription factor Hand2 in uterine stromal cells, suppressing the production of several fibroblast growth factors that mediate estrogen's effects on the epithelium. In mouse uteri lacking Hand2, continued production of these growth factors in the stroma maintained epithelial proliferation and stimulated estrogen-induced pathways, thus impairing implantation of embryos.
Hampton T. Progesterone’s Uterine Effects. JAMA. 2011;305(13):1290. doi:10.1001/jama.2011.387