The major contribution of the amniotic fluid after 20 weeks of gestation is :
After 20 weeks as the fetal skin becomes keratinized process of diffusion is impaired. In the second half of pregnancy, the fetal kidneys become active and thereafter fetal urine becomes the main source of amniotic fluid. (refer pg no:211 Sheila 2nd edition)
Ultrafiltrate of maternal plasma
Fetal urine
Fetal lung fluid
Fetal skin
General obstetrics