At 39 weeks of pregnancy, a woman’s fetal biparietal diameter is usually about 9.3cm. Biparietal diameter refers to the length of the widest part of the fetal head between the right and left sides, and is a common indicator used to observe the development of the fetus. When a woman is 39 weeks pregnant, the fetus has basically matured, and the biparietal diameter of the fetus is usually about 9.3cm. If the deviation between the biparietal diameter and 9.3cm at 39 weeks is relatively large, the fetus may have abnormal development, and it is generally necessary to further measure the head circumference, abdominal circumference and femur length of the fetus, as well as actively monitor the fetal movement, in order to comprehensively assess whether the growth and development of the fetus is normal or not.