Low amniotic fluid in 39 weeks of pregnancy may be caused by placental hypofunction, fetal malformation, amniotic membrane pathology, maternal factors and so on. 1. Placental hypoplasia. Degenerative changes of the placenta can lead to placental hypofunction, fetal growth restriction, chronic fetal hypoxia, causing fetal blood redistribution. In order to ensure the blood supply to the fetal brain and heart, renal blood flow decreases, fetal urine production decreases, resulting in a decrease in amniotic fluid. 2. Fetal malformation. Abnormalities in the structure of the fetal urinary system in late pregnancy cause oliguria or anuria, which can lead to a decrease in the amount of amniotic fluid. 3. Amniotic lesions. Certain unexplained decreases in amniotic fluid are associated with changes in the permeability of the amniotic membrane as well as inflammation and intrauterine infection. Rupture of the membranes and leakage of amniotic fluid at a rate faster than the rate of production of amniotic fluid can lead to low amniotic fluid. 4. Maternal factors. Hypertension during pregnancy, hypothyroidism, or pregnant women taking drugs such as indomethacin, diuretics, and angiotensin-converting enzyme inhibitors may lead to amniotic fluid hypotension. Low amniotic fluid in late pregnancy should observe the fetal heart condition, clarify the cause of the disease in a timely manner, and actively deal with it, so as to avoid low amniotic fluid causing intrauterine hypoxia of the fetus.