= Progesterone
<Progesterone> is a <steroid hormone> produced prominently by the <corpus luteum> after <ovulation> and by the <placenta> during <pregnancy>. It helps transform and maintain the <endometrium> in a state that supports <implantation> and subsequent <pregnancy>. Its withdrawal, alongside other hormonal changes, contributes to <menstruation> when <pregnancy> is not established.<Biology 2e by Mary Ann Clark and others>[Section 43.4, ovarian and uterine hormonal control]{ref}
Like other steroid signals, <progesterone> can act through a <nuclear receptor> to change <gene expression>. Its effect is therefore not merely the presence of a substance in <blood>: transport, <receptor> availability and <tissue> state shape the response. A post-ovulatory <progesterone> rise and an estrogen-driven proliferative phase describe different parts of the coordinated ovarian and uterine cycle.
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