The reasons for the formation of large biparietal diameters include miscalculation of the gestational week, over-nutrition, gestational diabetes and other reasons, which need to be analyzed specifically. 1. Miscalculation of gestational week: Some women have irregular menstruation and cannot clearly determine the specific gestational week when they are pregnant, which will lead to the biparietal diameter of the fetus not matching with the gestational week they calculated. 2. Over-nutrition: If the pregnant woman’s diet is high in nutrients, and the pregnant woman’s exercise is on the low side, it will lead to over-nutrition of the fetus, which will develop more rapidly and lead to a larger biparietal diameter. 3. Gestational diabetes: If the pregnant woman has gestational diabetes, it will also put the child in a state of high sugar, which will make the child’s growth too fast and lead to a large biparietal diameter. If the child’s biparietal diameter is enlarged, it is recommended to go to the hospital in time, and under the guidance of the doctor to take appropriate methods of intervention.