A biparietal diameter of 9.1cm at 34 weeks is considered normal. The average value of the biparietal diameter of a fetus at 34 weeks of pregnancy is 8.61±0.63cm, therefore, it is within the normal range if the fetal biparietal diameter is detected at 9.1cm at 34 weeks of pregnancy. The length of the widest part of the fetal head between the right and left sides is the length of the biparietal diameter. If the biparietal diameter is enlarged, an obstetric ultrasound or MRI is needed to check for craniofacial structures. If the biparietal diameter is simply enlarged, but no intracranial structural abnormality is found, and there is no abnormality such as hydrocephalus or widening of the lateral ventricles, it is also normal. In the late stage of pregnancy, it is mainly necessary to review the obstetric ultrasound on a regular basis to assess the size of the fetus, fetal position, length of head diameter such as the biparietal diameter and the pelvic diameter of the pregnant woman, to see if it is suitable for a normal delivery, and if the biparietal diameter of the fetus is too large, it may be necessary to carry out a caesarean section.