After 16 weeks of gestation, the dominant source of amniotic fluid volume, and the dominant route of its removal, are respectively:

• A Fetal urination; fetal swallowing ✓
• B Transudation across fetal skin; evaporation through the membranes
• C Fetal lung liquid secretion; reabsorption through the chorion laeve
• D Maternal plasma ultrafiltration; active transport by the amnion into fetal vessels

Explanation

From the second trimester onward, fetal urine becomes the major contributor to amniotic fluid volume, and fetal swallowing is the major route of removal, together maintaining dynamic balance. Before keratinization of fetal skin, transudation contributed significantly, but that ceases as skin cornifies around 20 weeks. Obstruction of either limb of this cycle produces oligohydramnios, as in renal agenesis, or polyhydramnios, as in gastrointestinal obstruction impairing swallowing.

Reference: Williams Obstetrics, 26th ed.

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Written and medically reviewed by the StethoPrep medical team.