The causes of large biparietal diameter ratio may include lack of exercise of the pregnant mother, over-nutrition, fetal craniocerebral malformation and so on. 1. Lack of exercise of pregnant mothers: The biparietal diameter of the fetus is large, mainly because of the lack of exercise of pregnant mothers. If you sit or lie down for a long time during pregnancy, it will cause the biparietal diameter to be big, so you should do more exercise and walk more during pregnancy. 2. Excessive nutrition: Excessive nutrition of pregnant mothers is also a major cause of large fetal biparietal diameter. If pregnant women eat too much during pregnancy and the fetus develops well, then the biparietal diameter will be large. 3. Fetal craniocerebral malformation: Fetal craniocerebral malformation will cause the whole head of the fetus to be enlarged and irregular in shape. Ultrasound will usually show that the biparietal diameter of the fetus is large. If the biparietal diameter of the fetus is large, it is recommended to go to a regular hospital for systematic examination.