Which of the following structures is formed from the proliferation of chorionic villi adjacent to the decidua basalis?

Amniotic fluid volume is tightly regulated and serves multiple functions, including cushioning the fetus, facilitating movement, and allowing lung development. In the third trimester, the majority of amniotic fluid is produced by fetal urine excreted into the amniotic cavity and lung fluid secreted by the fetal respiratory tract. Normal amniotic fluid volume ranges between 500 to 1000 mL at term. Fluid turnover involves swallowing and absorption by the fetus as well as contributions from membranes and maternal sources.

Rationale for correct answers

C. Fetal urine production significantly increases in the third trimester, contributing the bulk of amniotic fluid volume. Additionally, fetal lungs secrete fluid that mixes with the amniotic fluid, ensuring appropriate volume and composition. This combined secretion accounts for the majority of amniotic fluid after 28 weeks gestation.

Rationale for incorrect answers

A. Diffusion of fluid from maternal blood across the placental membranes contributes minimally to amniotic fluid volume, especially in the third trimester, as fetal urine becomes predominant.

B. Secretions from the fetal gastrointestinal tract do not contribute significantly to amniotic fluid volume; rather, swallowed amniotic fluid is absorbed, not secreted into the cavity.

D. Transudation from fetal skin is significant only early in gestation before keratinization, which is complete by about 20 weeks, greatly reducing this source by the third trimester.

Take home points

• The majority of third trimester amniotic fluid is produced by fetal urine and lung fluid secretions.
• Amniotic fluid volume ranges from 500 to 1000 mL near term and is critical for fetal development.
• Maternal diffusion and fetal skin transudation contribute minimally after mid-gestation.
• Fetal swallowing and absorption regulate amniotic fluid volume alongside production.

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.