Progesterone nervous tissue, especially in the brain, and in adipose (fat) tissue, as well. During human pregnancy, progesterone is produced in increasingly high amounts by the ovaries and placenta. At first, the source is the corpus luteum that has been "rescued" by the presence of human chorionic gonadotropin (hCG) from the conceptus. However, after the 8th week, production of progesterone shifts to the placenta. The placenta utilizes maternal cholesterol as the initial substrate, and most of the produced progesterone enters the maternal circulation, but some is picked up by the fetal circulation and used as substrate for fetal corticosteroids. At term
Menstrual cycle maturity is called a tertiary or Graafian follicle, and it contains the ovum. Ovulation is the second phase of the ovarian cycle in which a mature egg is released from the ovarian follicles into the oviduct. During the follicular phase, estradiol suppresses release of luteinizing hormone (LH) from the anterior pituitary gland. When the egg has nearly matured, levels of estradiol reach a threshold above which this effect is reversed and estrogen stimulates the production of a large amount of LH. This process, known as the LH surge, starts around day 12 of the average cycle and may last 48