The so-called 39-week biparietal diameter actually refers to the maximum diameter of the fetal skull at 39 weeks of pregnancy. Normally, a woman’s fetal biparietal diameter can reach about 96 mm at 39 weeks of pregnancy, but due to the individual growth and development of the fetus, the size of the fetal biparietal diameter can vary somewhat. Therefore, at 39 weeks of pregnancy, as long as the fetal biparietal diameter size is between 93 and 98 mm, it is usually considered to be a normal fetal size. This is because the size of the fetus may vary from one week to the next during the middle and late stages of pregnancy due to the different nutritional intake of each woman, as well as the different growth and development of the fetus and the early and late implantation of the fertilized egg. However, if at 39 weeks of pregnancy, the size of the fetus’ biparietal diameter differs by more than one week from the actual gestational week, it should be considered as fetal growth failure, also known as fetal intrauterine growth restriction, or fetal overgrowth, which may be caused by too much nutritional intake of the pregnant woman, or may be caused by gestational-diabetes. Therefore, in late pregnancy, it is usually necessary to review the ultrasound regularly to see the growth and development of the fetus. If the fetus is too large, it is usually necessary to control the diet to control the growth of the fetus in order to have a better delivery.