 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.^[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.