Self-Assessment Questions

19. The steroid hormones of pregnancy

• What is the rate-limiting factor in placental estrogen production?

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  Correct answer:
  The placenta contains large quantities of the enzyme aromatase, which aromatizes androgen precursors into estrogens. Placental aromatase activity is so robust that it is never rate limiting. Rather, the rate at which estrogens are produced is dependent on the supply of androgen precursors to the placenta. The majority of these precursors (mostly dehydroepiandrosterone sulfate) come from the maternal adrenal gland via uteroplacental blood flow; a minority comes from the fetus.