[Estimate of fetal long bone length in early pregnancy: comparison between mathematical formulae]

The aim of this study was to determine the efficacy of different mathematical formulae described in the literature and to propose new mathematical formulae to estimate fetal long bones biometry in early pregnancy.

In 1481 singleton euploid fetuses a transvaginal ultrasound examination was performed between 9 and 16 week's gestation. To determine the relationship between the biparietal diameter and long bone lengths, a sample group of 100 randomly chosen normal fetuses was evaluated by regression equations. The equations derived were then tested in the remaining 1381 control fetuses and the mean absolute percentage error and the mean systematic error with their standard deviations were compared with previously reported mathematical formulae by other authors.

All previous formulae described in literature when applied to our population revealed an overestimation of expected long bones measurements. Using our mathematical formulae, the mean absolute percentage error and the mean systematic error were 11.5% and 1% in estimating femur and 10.9% and 0.8% in estimating humerus length, respectively.

These newly derived ultrasound morphometric formulae could be proposed as reliable models in estimating fetal long bone lengths in early pregnancy.