Appropriate age for dating formula
Some clinicians use the unweighted mean of the 4 most commonly used biometric parameters ( biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC), and femur length (FL) ) to establish a gestational age  Ultrasound Estimate of Fetal Weight In practice the most common equations for calculating the estimated fetal weight (EFW) are reported to be the Shepard and Hadlock formulas [5,8,9]: Shepard: Log 10 (weight) = -1.7492 0.166*BPD 0.046*AC - 2.646*(AC*BPD)/1,000 Hadlock 1: Log 10 (weight) = 1.304 0.05281*Ac 0.1938*FL -0.004*AC*FL Hadlock 2: Log 10 (weight) = 1.335-0.0034*AC*FL 0.0316*BPD 0.0457*AC 0.1623*FL Hadlock 3: Log 10 (weight) =1.326-0.00326 *AC*FL 0.0107*HC 0.0438*AC 0.158*FL Hadlock 4: Log10 (weight) =1.3596 -0.00386* AC * FL 0.0064*HC 0.00061*BPD*AC 0.0424*AC 0.174*FL Regardless of the formula used the accuracy of the sonographic estimate of the EFW is affected by supoptimal imaging and biological variation . al., Estimating fetal age: computer-assisted analysis of multiple fetal growth parameters.
In addition the accuracy of the sonographic estimate decreases with increasing birth weight [26,27] , and tends to be overestimated in pregnancies suspected of being large for gestational age (LGA) and underestimated in pregnancies with preterm premature rupture of membranes (PPROM) and suspected fetal growth restriction (FGR) .
Estimate of Gestational Age The American College of Obstetricians and Gynecologists (ACOG) and the Society of Obstetricians and Gynaecologists of Canada recommend ultrasound measurement of the crown rump length (CRL) of the embryo or fetus as the most accurate method to establish or confirm gestational age [21,22]. al., Estimation of fetal weight with the use of head, body, and femur measurements--a prospective study.
It is recommended that crown-rump length be used up to 84 mm, and other parameters be used for measurements 84 mm the biparietal diameter (BPD) or head circumference (HC) is the best predictor of gestational age  . et al., Sonographic estimation of fetal age and weight.
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Very small and very large babies have higher mortality and morbidity rates than infants of normal size . al., Evaluating the thresholds of abnormal second trimester multiple marker screening tests associated with intra-uterine growth restriction.
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We cannot therefore recommended any change of current practice.