The standard value of fetal biparietal diameter at 31 weeks is 7.46-8.66 cm. The biparietal diameter is a very important indicator for the fetus, which can tell the development of the fetus and determine whether the mother can deliver successfully. If the biparietal diameter of the fetus is larger than the transverse pelvic exit diameter in late pregnancy, it will affect the natural delivery and need to be prepared for a cesarean section. Pregnant women should pay attention to more rest and control their diet in the second trimester to avoid the fetus being too large and affecting the delivery. It is also necessary to have appropriate activities in the second trimester, such as taking a walk, which is beneficial to the later stage of labor. In addition, do not eat foods with too much starch and do not eat too many eggs.