The principal estrogen of late pregnancy, estriol, cannot be synthesized by the placenta alone. The obligatory precursor that the fetus must supply is:

• A Pregnenolone secreted by the fetal corpus luteum
• B DHEA sulfate secreted by the fetal adrenal cortex ✓
• C Testosterone aromatized by fetal Sertoli cells
• D Cholesterol delivered via fetal LDL receptors

Explanation

The feto-placental unit works in series: the fetal adrenal cortex lacks 3-beta-hydroxysteroid dehydrogenase and therefore exports large amounts of DHEA sulfate, which the placenta converts via sulfatase and aromatase into estriol. The placenta also lacks 17-alpha-hydroxylase, so it cannot make its own androgen precursors. Cholesterol and pregnenolone serve progesterone synthesis, not estriol, so measuring maternal serum estriol reflects fetal adrenal function.

Reference: Ganong's Review of Medical Physiology, 26th ed.

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