The average biparietal diameter of a fetus at 17 weeks of pregnancy is 3.97±0.44cm, with variations between individuals. Fetal biparietal diameter is the length of the widest part of the fetus between the two sides of the head, the average biparietal diameter of the fetus at 17 weeks of pregnancy is 3.97 ± 0.44cm, there are individual differences in the fetus, the biparietal diameter will also appear different data. In different gestational weeks of the fetal body of each diameter measurement value is different, gestational weeks of different fetal growth and development rate is not the same, in pregnancy check the biparietal diameter, can reflect the shape of the fetal cranial brain, but can not be used as an absolute basis for judging the size of the cranial brain, through the biparietal diameter check the existence of abnormalities, combined with the results of the head circumference to judge. After pregnancy, do a good job of pregnancy obstetric examination, there are abnormalities further processing.