At what point during pregnancy does fetal urine become the primary source of amniotic fluid?

Fetal urine becomes the primary source of amniotic fluid in the middle of the second trimester, around 20 to 24 weeks of gestation. During the first trimester, amniotic fluid primarily originates from maternal sources and transudate from the fetal skin. As the pregnancy progresses and the fetus develops, particularly around the 16 to 18-week mark, the kidneys begin to produce urine. By the middle of the second trimester, the production of fetal urine increases significantly, making it the dominant contributor to the volume of amniotic fluid.

This shift is crucial for normal fetal development, as adequate amniotic fluid is necessary for proper lung development, movement, and cushioning of the fetus. By later stages of pregnancy, the fetus's urine production continues to play a critical role in maintaining amniotic fluid levels, but the transition point occurs around the middle of the second trimester.