Which of the following best describes the purpose of amniocentesis?

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.

Oligohydramnios is defined as a decreased amount of amniotic fluid, usually less than 500 mL in the third trimester or an amniotic fluid index (AFI) less than 5 cm. It results from decreased fetal urine production or increased fluid loss and is associated with fetal renal anomalies, rupture of membranes, or placental insufficiency. The amniotic fluid plays critical roles in lung development, cushioning, and movement. Prolonged oligohydramnios compromises lung growth leading to pulmonary hypoplasia, a condition where the lungs are underdeveloped, impairing neonatal respiratory function.

Rationale for correct answers

C. Pulmonary hypoplasia occurs because amniotic fluid is essential for normal lung development by maintaining fluid pressure and allowing lung expansion in utero. Prolonged oligohydramnios reduces this fluid, resulting in underdeveloped lungs with fewer alveoli and decreased pulmonary vasculature, causing respiratory insufficiency at birth.

Rationale for incorrect answers

A. Macrosomia, defined as fetal weight above 4000 grams, is unrelated to oligohydramnios and more often linked to maternal diabetes or genetic factors. Reduced amniotic fluid does not cause fetal overgrowth.

B. Shoulder dystocia is an obstetric complication caused by fetal size disproportion, often macrosomia, and is not directly caused by oligohydramnios.

D. Hyperbilirubinemia results from increased red blood cell breakdown or impaired conjugation, unrelated to amniotic fluid volume. Oligohydramnios does not predispose to bilirubin metabolism disorders.

Take home points

• Oligohydramnios is characterized by AFI less than 5 cm or fluid volume under 500 mL in the third trimester.
• Prolonged oligohydramnios leads to pulmonary hypoplasia due to impaired fetal lung development.
• Macrosomia and shoulder dystocia are unrelated to oligohydramnios but linked to fetal size.
• Hyperbilirubinemia is unrelated to amniotic fluid volume but linked to red blood cell metabolism.