After 20 weeks of gestation, what primarily produces amniotic fluid?

The production of amniotic fluid after 20 weeks of gestation is primarily due to fetal excretion of urine. As the fetus develops, the kidneys begin to produce urine, which is a significant contributor to the volume of amniotic fluid. The fetal urine becomes the primary source of amniotic fluid during the later stages of pregnancy, especially as gestation progresses past the halfway mark.

While maternal serum is a source of initial amniotic fluid, particularly in the first trimester, it is not the main contributor as pregnancy advances. Similarly, while there may be minimal contributions from the fetal respiratory tract, these do not play a major role compared to urine production. Lastly, maternal intake of hydration influences overall fluid balance and may impact amniotic fluid levels to some extent, but it does not directly produce amniotic fluid. Hence, fetal urine production is the most significant factor in the formation of amniotic fluid after 20 weeks of gestation.