Appropriate dating age formula
Correct evaluation depends on the accuracy of the gestational age being used , the precision of the weight measurements , and using a weight curve that represents the population being studied. 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. 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. Because early detection of growth abnormalities may help to prevent fetal demise and manage perinatal complications more appropriately , monitoring of fetal growth is an important part of antepartum care. The suggested results are not a substitute for clinical judgment.
However, there is not universal agreement on these definitions , and some have proposed the 15th and 97th percentiles as more optimal cut-offs to define SGA and LGA respectively . Many regression equations are available using various combinations of parameters to estimate the gestational age. 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. Table 2 shows data from the National Institute of Child Health and Human Development (NICHD) Fetal Growth Studies which demonstrates the differences in fetal growth between different races and ethnicities  . American College of Obstetricians and Gynecologists. Not all agree that the use of custom growth charts improves prediction of perinatal mortality [6,7, 52] .