The formula for estimating the size of a fetus based on its biparietal diameter is generally the weight of the fetus in grams = 900 x biparietal diameter – 5200. Biparietal diameter refers to the distance between the top bulges on each side of the fetal head, which is the widest distance in the vertical radius of the fetal cranial mid-septum. Generally, women measure the biparietal diameter of the fetus in the middle and late stages of pregnancy to understand the development of the fetus in the uterus and also to estimate the weight of the fetus. Estimating fetal weight based on the biparietal diameter is generally based on the formula, that is, fetal weight (grams) = 900 x biparietal diameter – 5200. This calculation is an approximate estimation, and the actual weight of the fetus after birth may be different from the calculated weight, which is generally within the error of 10-15%.