Which formula you use for ultrasound estimation of fetal gestation matters

In a Swedish study three different formulae were applied to mid-trimester ultrasound measurements of fetal biparietal diameter and femur length in order to calculate gestational age in infants less than 27 weeks gestation. These estimates were compared to gestation determined by last menstrual period and to clinical estimates of gestation. Two of these formulae yielded results quite different from the third one. Many infants estimated to be at 23 weeks gestation by one method of estimation were classified as being at 22 weeks if a different method was used, and vice versa.

Effect of various dating formulae on sonographic estimation of gestational age in extremely preterm infants. Simic et al 2012.