When does fetal urine become the primary source of amniotic fluid?

Fetal urine becomes the primary source of amniotic fluid by the middle of the second trimester, typically around 20 weeks of gestation. Before this point, amniotic fluid is primarily produced through the transudation of maternal serum and the secretion from the fetal membranes. As the fetus develops, particularly as the kidneys begin to function, fetal urine production significantly increases. By approximately 20 weeks, the kidneys are mature enough to produce urine, which is excreted into the amniotic sac and plays a crucial role in both cushioning the fetus and facilitating lung development through inhalation of the fluid. This transition marks a significant developmental stage in pregnancy, highlighting the importance of renal function in fetal well-being and the dynamics of amniotic fluid regulation.