when does fetus make urine?
Amniotic fluid is present in the amniotic sac. It is generated from maternal plasma, and passes through the fetal membranes by osmotic and hydrostatic forces. When fetal kidneys begin to function in about week 16, fetal urine also contributes to the fluid. The fluid is absorbed through the fetal tissue and skin. After the 20th-25th week of pregnancy when the keratinization of an embryo's skin occurs, the fluid is primarily absorbed by the fetal gut.