Amniotic fluid of 40mm in the 4th month of pregnancy is normal. The amount of amniotic fluid during pregnancy is mainly measured by ultrasound. Ultrasound during pregnancy can be diagnosed as excessive amniotic fluid if it suggests that the maximum depth of amniotic fluid is greater than 8cm, or the amniotic fluid index is greater than 25cm. An ultrasound indicating that the depth of amniotic fluid is less than or equal to 2 cm or the amniotic fluid index is less than or equal to 5 cm can be diagnosed as too little amniotic fluid. The amount of amniotic fluid during pregnancy increases with the week of pregnancy, mainly from the mother’s serum in early pregnancy and mainly through the fetus’ urine after the middle of pregnancy. 38 weeks, the amount of amniotic fluid can reach 1000ml, after that the amount of amniotic fluid gradually decreases, at 40 weeks the amount of amniotic fluid can be 800ml, and then gradually decreases, even down to less than 300ml. If too much or too little amniotic fluid is found during pregnancy, the fetus should be investigated for malformations.