The fetal biparietal diameter is an essential factor in calculating the weight of the fetus, which is about 4.88 cm at 20 weeks of pregnancy, but there are individual differences in the size of the biparietal diameter, which is influenced by various aspects such as genetics and nutrition, and is not absolute. If the fetal biparietal diameter is small, the common reasons are as follows: 1, due to irregular menstrual cycle of pregnant women caused by delayed ovulation, so that the fertilized egg is late, resulting in the fetal biparietal diameter than the actual pregnancy week is small, at this time generally do not need special treatment, regular maternity check-ups can be; 2, pregnant women in pregnancy during the intake of nutritional deficiencies or by disease factors, resulting in fetal growth and development, the biparietal diameter is small. At this time, attention should be paid to adjust the daily diet of pregnant women, more nutritious, easy to digest food, and at the same time pay attention to the treatment of the original disease. If the fetal biparietal diameter is found to be large by ultrasound during pregnancy checkup, it is mostly due to the following reasons: 1, the last menstrual record of the pregnant woman is not accurate, resulting in the calculation of the fetal biparietal diameter of the corresponding pregnancy week is small, while the actual fetal biparietal diameter is large, at this time, regular maternity checkup is needed; 2, the nutritional absorption during pregnancy is better, at this time, attention should be paid to adjust the pregnant woman’s diet, supplemented by appropriate exercise; 3, genetic factors, the couple The head diameter of both partners is relatively large, no special treatment is needed at this time; 4, fetal hydrocephalus with ventricular widening, at this time should go to the hospital in time, under the guidance of the doctor to deal with, serious cases may need to induce labor. The double parietal diameter can not only determine the growth and development of the fetus, but also is an important factor in whether the delivery can be carried out, if the double parietal diameter is too large, a cesarean delivery may be required.