The ideal fetal head circumference calculation.
Various formulas have been employed to compute the circumference of an ellipse. These formulas can be separated into three groups: formulas for a circle, formulas for an incorrect ellipse, and a very complicated formula for a correct ellipse. The errors caused by the first two groups are termed systematic because they occur every time that one of these formulas is used. When measuring fetal head circumference, the errors increase as the fetal head is more ellipsoid, becoming more than 1% when the ratio of the biparietal to fronto-occipital diameter decreases to less than .70. Near term, when the fetal head is large, this could create an error in predicting gestational age of almost .8 weeks. The third group is associated with mathematical random errors because of the difficulty in using a cumbersome equation. All of these formulas and their shortcomings are analyzed. A calculation for the correct circumference of an ellipse is proposed using the simple formula for a circle times a correction factor. The correction factor depends solely on the ratio of the biparietal to fronto-occipital diameter. This formula should minimize random errors and eliminate systematic errors in the calculation of fetal head circumference and will permit a more accurate analysis of fetal age.