The normal value of biparietal diameter at 27 weeks of pregnancy is 71.1mm, and the details vary from person to person. At 27 weeks of gestation, the normal value of the fetal biparietal diameter is 71.1mm, but there are individual differences in the developmental rate of the fetus, so clinically it is generally considered that the size of the biparietal diameter within two standard deviations above and below the normal value is considered to be normal, with a reference range of 64.8mm to 77.4mm. If the size of fetal biparietal diameter at 27 weeks of gestation exceeds the normal reference range, it is necessary to check whether the calculation of gestational week is wrong, to exclude the existence of gestational diabetes mellitus, fetal hydrocephalus, etc. If the size of fetal biparietal diameter is lower than normal, to exclude the error in the calculation of gestational week, and at the same time, it should be further clarified whether there is growth restriction of the fetus. If the developmental abnormality of the fetus is determined, timely medical treatment should be sought to avoid delaying the condition and leading to adverse pregnancy outcomes.