A biparietal diameter of 7.5 is approximately 30 weeks. The biparietal diameter is the distance between the two parietal bulges of the fetus and is the largest transverse diameter of the fetal head. The size and position of the fetus, as well as the presence or absence of abnormalities, determine the ease or difficulty of delivery. The head of the fetus is the largest part of the fetal body, and an oversized head during labor can make it difficult for the fetus to pass through the birth canal. The biparietal diameter of the fetus can be measured by ultrasound as a reference for estimating the week of gestation. Generally speaking, at 30 weeks of gestation, the fetal biparietal diameter is 7.83 s 0.62 cm. Measurements of fetal head circumference, abdominal circumference and femur length can also be taken to estimate the week of gestation. When the growth of the fetus is restricted, or when the growth of the fetus is abnormal, the biparietal diameter may be different from the normal value.