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].
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.
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..." . Neither nor any other party involved in the preparation or publication of this site shall be liable for any special, consequential, or exemplary damages resulting in whole or part from any user's use of or reliance upon this material. Duryea EL,et al., A revised birth weight reference for the United States. 2014 Jul;124(1):16-22.doi:10.1097/AOG.0000000000000345. Measurement of the symphysis-fundal height (SFH) is a common screening method used to estimate the gestational age and fetal growth after 24 weeks gestation. Further trials are needed."  When there are factors that increase the risk for intrauterine growth restriction (IUGR) or the SFH is unreliable because of maternal obesity, twin pregnancy, polyhydramnios, or the presence of uterine leiomyomas (fibroids) ultrasonography may be a better screening modality for growth problems in the fetus. Sonographic evaluation of intrauterine growth retardation. Prenatally the sonographically estimated fetal weight is used together with weight tables to evaluate fetal growth.