There are many indicators for calculating the size of a third trimester fetus, and there are individual differences in the exact size of the fetus. Third trimester fetus corresponds to the fetus at the end of the 12th week of gestation, at this time the length of the fetus is about 9cm, the parietal hip length is about 6-7cm, the biparietal diameter is about 2.1cm, the transverse cerebellar diameter is about 0.82cm, the circumference of the head is about 7.65cm, the circumference of the abdomen is about 6.03cm, there may be individual differences, it is generally considered to be within the standard range are normal. If the size of the fetus at the end of the 12th week of pregnancy is significantly larger than the level of the same pregnancy, it is necessary to check the expected date of delivery again, to determine whether there is any error in the gestational week, and to clarify whether the mother has diabetes or other conditions. If the size of the fetus is significantly lower than the level of the same pregnancy, correct the gestational week, but also to rule out the existence of fetal growth and developmental restrictions. When abnormalities occur in the fetus, pregnant women should seek timely medical attention to avoid delays that could lead to adverse pregnancy outcomes.