Daily blood samples were taken for progesterone (P) and estradiol (E2) measurements from women who showed a platelet response consistent with the presence of viable embryos after in vitro fertilization and embryo transfer procedures. A comparison of steroid levels between those women who became pregnant and those who did not revealed the following: at and after the time of transfer, women who failed to become pregnant had significantly higher E2 levels and a lower ratio of P/E2 than women who became pregnant. The P/E2 ratio was a better predictor of implantation failure than was the absolute level of either hormone. Experiments were done in mice to test the hypothesis that P could protect implantation of the embryo against the inhibitory effects of high E2. In mice, implantation was inhibited by relatively high levels of E2. This effect was overcome by concomitant administration of P. There was a significant dose-response-related interaction of P with the E2.
Gidley-Baird, A. A., O’Neill, C., Sinosich, M. J., Porter, R. N., Pike, I. L., & Saunders, D. M. (1986). Failure of implantation in human in vitro fertilization and embryo transfer patients: The effects of altered progesterone/estrogen ratios in humans and mice. Fertility and Sterility, 45(1), 69–74. https://doi.org/10.1016/0020-7292(87)90249-9