The major contribution of the amniotic fluid after 20 weeks of gestation is :
In early gestation, amniotic fluid mainly comes from maternal plasma and fetal urine. But after 20 weeks, the primary source shifts. I remember that by the second trimester, fetal urine becomes the main contributor. However, after 20 weeks, maybe there's another shift? Wait, no, actually, after 20 weeks, the fetal kidneys are mature enough to produce significant urine, which becomes the main component. But wait, I think there's a point where the contribution from fetal lungs and other secretions might play a role. Hmm.
Wait, the key here is that after 20 weeks, the majority of amniotic fluid comes from fetal urine. Before 20 weeks, it's mostly from amnion and maternal serum, but as the fetus develops, the fetal kidneys take over. So the correct answer should be fetal urine. Let me confirm. Yes, after 20 weeks, fetal urine is the major source. Other options like fetal skin, maternal serum, or fetal lungs are less significant. The skin contributes earlier, but after 20 weeks, the skin is more permeable, allowing fluid to pass through. So the main contributors after 20 weeks are fetal urine and fetal respiratory secretions. Wait, but which is more significant? I think fetal urine is the primary contributor. So the correct answer is fetal urine. The other options would be incorrect because maternal serum decreases as the placenta becomes more efficient, fetal lungs contribute some secretions but not the majority, and fetal skin stops being a major source once the epidermis thickens. So the answer is fetal urine.
**Core Concept**
Amniotic fluid composition shifts during gestation. After 20 weeks, fetal urine becomes the primary contributor due to mature fetal kidney function, while maternal serum and fetal respiratory secretions play minor roles.
**Why the Correct Answer is Right**
After 20 weeks, fetal kidneys produce urine at a rate of ~400–500 mL/day, which accounts for ~50–70% of amniotic fluid volume. This fluid is reabsorbed by the fetal gastrointestinal tract and placenta, maintaining equilibrium. The high contribution of urine reflects fetal renal maturity and is critical for lung development via mechanical distension.
**Why Each Wrong Option is Incorrect**
**Option A:** Fetal skin contributes minimally after 20 weeks, as the stratum corneum thickens, reducing transudation.
**Option B:** Maternal serum input declines after mid-gestation due to placental barrier efficiency.
**Option D:** Fetal respiratory secretions (e.g., from the trachea and lungs) contribute ~10–20% but are not the major source.
**Clinical Pearl**
Oligohydramnios after 20 weeks may indicate fetal renal agenesis (Potter sequence), as urine is the primary fluid source. Polyhydramnios can result from maternal diabetes or fetal swallowing defects.
**Correct Answer: B. Fetal urine**