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Progesterone is a steroid hormone secreted in women by the corpus luteum and the follicle in normal menstruation, and by the placenta in pregnancy. It is also synthesised by the testes and adrenal cortex in small amounts.
The action of progesterone is in the preparation of the uterus for the fertilised ovum; thus it supports the secretory phase of the menstrual cycle.
In pregancy it prepares the breast for lactation, and acts on the myometrium to reduce its sensitivity to oestrogens and enhance the conversion of oetradiol to less active metabolites.