It is not necessarily due to low amniotic fluid, but may also be caused by inappropriate pregnancy, pregnant women’s abdominal wall is too thick and so on. 1. Low amniotic fluid: too little amniotic fluid is likely to cause the fetus to be curled up in the abdomen, not free to turn over and other activities, so it is possible to see the fetus clearly. 2. inappropriate pregnancy: because too early or too late for four-dimensional ultrasound may not be able to see the situation of the fetus, if earlier will appear fetal organ development is incomplete, so can not see the fetus, too late may appear larger fetus can not see the fetus limbs of the situation. 3. pregnant women abdominal wall is too thick: pregnant women abdominal wall is too thick will cause four-dimensional ultrasound irradiation difficulties, affecting the quality of the image, can not see the fetus. Four-dimensional image can not see clearly there may be other reasons, if four-dimensional image can not see clearly fetal situation should actively cooperate with the doctor, under the guidance of the doctor to carry out the appropriate treatment.