At 27 weeks of gestation, the biparietal diameter of the fetus is between 6.9 and 7.1cm, but the exact size of the fetus is also related to maternal nutrition. The biparietal diameter of the fetal head is the length between the right and left sides of the fetal head, and it is used to observe the development of the child and to determine whether the child can be delivered successfully. At 27 weeks of gestation, the biparietal diameter of the fetus is between 6.9 and 7.1cm. However, the exact size is related to the nutritional absorption of the mother. If the mother’s nutritional absorption is average in the early stage of pregnancy due to vomiting or other reasons, the child’s biparietal diameter may be 6.9cm, or even lower than 6.9cm, in which case the mother needs to strengthen her own nutrition. If the mother’s nutritional status is good, or even the emergence of gestational diabetes mellitus, may lead to fetal biparietal diameter is too large, need to control the diet, to prevent the emergence of huge children. Pregnant women are advised to pay attention to their health care during pregnancy, to take regular checkups, and to seek timely medical attention if any abnormalities occur.