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The SOGC recommends the earliest ultrasound with a crown rump length equivalent to at least 7 weeks (or 10 mm) should be used to determine the gestational age.
The SOGC also recommends "...either the best CRL or the average of several satisfactory measurements should be used." The ACOG recommends "The measurement used for dating should be the mean of three discrete CRL measurements when possible..." .
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.
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.
Monochorionic twins growth rate growth begins to slow at 28 weeks and averages about 170 grams per week thereafter until 37 weeks  After obtaining a reliable gestational age and best estimate of the fetal weight, a growth chart or weight table may be used to assign an EFW growth percentile .
The sensitivity of SFH measurement for detecting abnormal intrauterine growth was less than 35% in one study . found that the sensitivity of SFH measurement for detecting fetal growth abnormalities could be improved by serial plotting of the SFH on customized charts . Hadlock FP, et al., In utero analysis of fetal growth: a sonographic weight standard. ACOG advises newborns whose birth weight is less than the 10th percentile for gestational age are considered small for gestational age (SGA) . Whereas a fetus with a weight that is less than the 10th percentile for its gestational age is considered to have fetal growth restriction (FGR), also known as intrauterine growth restriction (IUGR) [5, 7]. The distance from the top of the pubic bone (symphysis pubis) to the top of the pregnant uterus (fundus) is measured in centimeters (cm). The SFH in centimeters should be equal to the gestational age in weeks.