The first check of amniotic fluid at 18 weeks of pregnancy 4 cm the second 3.6 cm may be normal, or it may be related to the development of the fetus. 1. Normal situation: If you usually drink less water in the course of pregnancy, it may lead to a decrease in body fluids, and therefore the symptom of less amniotic fluid than the previous one may occur. 2. Abnormal fetal development: some fetuses in the development process of fetal structural abnormalities, changes in the function of the placenta, or pregnant women with maternal disease, may lead to a decrease in amniotic fluid, which is still within the normal range, but if the decrease in amniotic fluid in the late stage is more serious, it is recommended to consult a doctor for examination in a timely manner. During pregnancy, more ultrasound tests are needed to understand the development of the fetus, and termination of pregnancy may be needed if necessary.