An ultrasound at 28 weeks shows severe oligohydramnios with absent fetal kidneys and pulmonary hypoplasia. The primary determinant of amniotic fluid volume in the third trimester that this case illustrates is:

• A Balance between fetal urine production and fetal swallowing ✓
• B Transudation across the amniotic membranes
• C Secretion by the fetal tracheobronchial tree
• D Maternal hydration status across the placenta

Explanation

After mid-gestation, amniotic fluid volume reflects fetal urine output as the major inflow balanced against fetal swallowing as the major outflow. Bilateral renal agenesis abolishes urine production, producing oligohydramnios, pulmonary hypoplasia and the Potter sequence. Lung liquid contributes to airway development rather than bulk amniotic volume, and maternal hydration has little effect once the kidneys are absent.

Reference: Nelson Textbook of Pediatrics, 21st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.