The three regions of the decidua include which of the following? Select all that apply

The chorion frondosum is the part of the chorion characterized by proliferation of vascularized villi adjacent to the decidua basalis, which contributes to the formation of the fetal placenta. These villi undergo extensive branching and vascularization to establish maternal-fetal exchange. In contrast, the chorion laeve is the smooth, avascular part away from the decidua basalis. Proper development of the chorion frondosum is essential for adequate placental function, supporting fetal nutrition and gas exchange throughout pregnancy.

Rationale for correct answers

B. The chorion frondosum forms from the proliferation of chorionic villi adjacent to the decidua basalis, developing into the functional fetal placenta with a dense vascular network enabling exchange of nutrients, gases, and waste. This structure is essential for sustaining fetal growth.

Rationale for incorrect answers

A. The chorion laeve, or smooth chorion, forms from villi on the side of the chorion away from the decidua basalis; these villi regress and become avascular, so it does not contribute to placental formation.

C. The yolk sac is an early embryonic structure involved in initial hematopoiesis and nutrient transfer but is not formed from chorionic villi and is distinct from the chorion.

D. The umbilical cord develops from the connecting stalk and contains blood vessels connecting fetus to placenta but is not formed by chorionic villi proliferation adjacent to the decidua basalis.

Take home points

• The chorion frondosum is the vascularized region of chorionic villi adjacent to the decidua basalis forming the fetal placenta.
• The chorion laeve is avascular and forms the smooth chorion on the opposite side of the placenta.
• Proper placental development depends on chorionic villi proliferation and vascularization in the chorion frondosum.
• The yolk sac and umbilical cord are distinct structures not formed from chorionic villi adjacent to decidua basalis.

Fetal growth restriction (FGR) is a condition where the fetus fails to achieve its genetically predetermined growth potential, often due to placental insufficiency, leading to chronic hypoxia and nutrient deprivation. Oligohydramnios, defined as an amniotic fluid index less than 5 cm or a single deepest pocket less than 2 cm, frequently accompanies FGR. Amniotic fluid volume depends primarily on fetal urine production, lung fluid secretion, and swallowing. Placental dysfunction, fetal renal anomalies, and altered fetal circulation can decrease urine output, reducing amniotic fluid and causing oligohydramnios.

Rationale for correct answers

A. Reduced fetal urine output occurs secondary to placental insufficiency, which causes hypoxia and redistribution of blood flow, limiting kidney perfusion and thus decreasing urine production—the major source of amniotic fluid in the third trimester.

C. Congenital anomalies of the fetal renal system impair urine formation or excretion, directly reducing amniotic fluid volume, as fetal urine constitutes up to 90% of amniotic fluid after 16 weeks gestation.

E. A shift in fetal blood flow away from the kidneys (known as blood flow redistribution or “brain-sparing”) prioritizes vital organs during hypoxia, reducing renal perfusion and urine output, thus contributing to oligohydramnios.

Rationale for incorrect answers

B. Increased fetal swallowing of amniotic fluid is not a typical compensatory mechanism in FGR; rather, swallowing usually maintains normal fluid balance and is not significantly increased in FGR or oligohydramnios.

D. Absence of functional kidneys (e.g., bilateral renal agenesis) leads to anuria and severe oligohydramnios, but this is a specific congenital anomaly rather than a common cause of FGR-related oligohydramnios.

Take home points

• Placental insufficiency decreases fetal renal perfusion, reducing urine output and amniotic fluid.
• Fetal renal congenital anomalies can impair urine production, leading to oligohydramnios.
• Fetal blood flow redistribution prioritizes brain over kidneys, reducing renal function.
• Oligohydramnios is a frequent finding in FGR and indicates compromised fetal well-being.