An amniotic fluid index of 5.5cm at 39 weeks of pregnancy is on the low side. Amniotic fluid index is a fetal ultrasound examination, the doctor will use the umbilical level line and abdominal white line as a sign to divide the uterus into four parts, each part measured the sum of the maximum depth of amniotic fluid is the amniotic fluid index. The normal value of amniotic fluid index at 39 weeks of pregnancy is between 8-25cm. This figure can reflect the amount of amniotic fluid during pregnancy and assess the development of the fetus. If the amniotic fluid index is ≤5cm it is too little amniotic fluid and if the amniotic fluid index is ≥25cm it is too much amniotic fluid. Abnormal amniotic fluid volume may be the fetus has malformation, placental disease, the mother has pregnancy complications, etc., need to further check the cause, under the guidance of the doctor to deal with. Pregnant women should pay attention to the maternity checkup in the late pregnancy, as well as pay attention to the changes of fetal movement, and if abnormalities are found, consult a doctor in time.