The mid-20th century phase coalesced around a hormone-driven view of uterine receptivity, where estrogen-sculpted endometrial priming creates a receptive milieu that is then secured by progesterone-facilitated luteal function. Across studies, researchers mapped estrogen-responsive uterine tissues, linked estradiol to molecular and metabolic changes, and developed practical assays to quantify estrogen potency via the immature mouse uterus, all contributing to a coherent narrative of endometrial preparation. Findings on relaxation-regulating factors showed how coordinated uterine quiescence supports embryo accommodation, while progesterone’s restoration of ovulatory cycles and corpus luteum activity underscored the luteal context for implantation readiness. Overall, the period emphasized receptor- and tissue-level responsiveness as the engine of receptivity and made systematic inquiries into how hormonal signaling translates into a capable endometrium for embryo adhesion.