Part of “CHAPTER 108 – THE MATERNAL-FETAL-PLACENTAL UNIT“

Fetal membranes consisting of amnion and chorion were originally thought to be inactive endocrinologically. The amnion is a thin structure (0.02–0.5 mm) and contains no blood vessels or nerves. However, the fetal membranes play important roles during pregnancy in the transport and metabolism of hormones and in the events that lead eventually to parturition.45 Thus, although fetal membranes apparently do not synthesize hormones de novo, they have extensive enzymatic capabilities for regulating steroid hormone metabolism. Some of these enzymes are 5α-reductase, 3β-hydroxysteroid dehydrogenase, Δ4,5-isomerase, 20α-hydroxysteroid oxidoreductase, 17β-hydroxysteroid dehydrogenase, aromatase, and sulfatase. Also, fetal membranes contain large quantities of arachidonic acid, the obligate precursor of prostaglandins. Furthermore, they contain phospholipase A₂ and other enzymes that stimulate the release of arachidonic acid from glycerophospholipids in the amnion or chorion.8