Biparietal diameter between 8.37cm and 9.63cm is normal for a full-term fetus. Fetal biparietal diameter is also known as the large transverse diameter of the head, referring to the length of the widest part of the left and right sides of the fetal head, the requirements for the fetal biparietal diameter are different in different trimesters, coupled with certain differences in the development of the individual fetus, so there is only an approximate range of normal for fetal biparietal diameter, there is no strict numerical value. Fetal biparietal diameter is one of the important indicators to monitor and measure the growth and development rate of the fetus in each trimester, but it is not the only indicator, in addition to the biparietal diameter, fetal femur length is also an important indicator. Once the fetal biparietal diameter and femur length is less than two standard deviations, it is recommended to pay more attention to further good examination, to pay more attention to further examination of the child, do a fetal cranial nuclear magnetic to observe whether there is any structural abnormality, except the possibility of microcephaly.