Biparietal diameter multiplied by 900 minus 5200 is a formula for estimating fetal weight, which is somewhat accurate, but also subject to a certain degree of error.
Calculating fetal weight using the formula related to biparietal diameter is only a method of fetal weight estimation, and it is not yet clinically possible to calculate fetal weight with complete accuracy.
This is because the proportions of the body parts of each fetus are unique. If the fetus is very standardized, then the formula is relatively accurate, but if the head of the fetus is slightly larger than the rest of the body, then the accuracy of the calculation will be reduced.
In clinical practice, the above formula is a relatively common method to calculate fetal weight, so it can be used as a reference, but there is a certain possibility of error.