Progesterone (P4) is a female sex hormone that is primarily produced in the corpus luteum in normally menstruating women, with small amounts also produced in the adrenal cortex.
Progesterone is essential for the preparation of the uterus for implantation by causing the uterine lining to thicken. During pregnancy, progesterone production continues to maintain the uterine lining, help nurture the developing fetus throughout the pregnancy, and to trigger the breasts to produce milk. Progesterone also acts in non-reproductive tissues, often in partnership with estradiol. Examples include estradiol reduction of bone resorption and progesterone stimulation of bone formation, and coordinated increases in nitric oxide activity to improve blood flow.
Abnormally low progesterone levels are observed in mid-luteal phase in females whom have disorders of ovulation. This luteal phase deficiency is associated with infertility and spontaneous abortion, and is estimated to occur in approximately 10% of infertile women. Low progesterone levels during the first 10 weeks of pregnancy are indicative of threatened abortion and ectopic pregnancy.