Question

The major contribution of the amniotic fluid after 20 weeks of gestation :

• A. Ultrafiltrate and maternal plasma
• B. Fetal urine
• C. Fetal lung fluid
• D. Fetal skin

Answer 1

The Correct Option is B

Step 1: The right way to think about this is by trimester. Up to roughly 20 weeks the membranes and the non-keratinised fetal skin allow free passage of water, so the fluid mirrors maternal plasma as an ultrafiltrate.
Step 2: Keratinisation of fetal skin around mid-gestation seals off that route, and the fetal kidneys and lungs take over as the working sources.
Step 3: Urine output from the fetal kidneys is the single largest contributor in the second half of pregnancy, far exceeding lung liquid output. Clinically, renal agenesis or posterior urethral valves cause oligohydramnios for exactly this reason.
Step 4: Hence after 20 weeks the answer is fetal urine.
\[\boxed{\text{Fetal urine}}\]