Monitoring fetal growth is a standard component of antenatal care. Investigators have developed several equations for estimating fetal weight in the late second and the third trimester. These equations involve a variety of sonographically obtained biometric measurements. The fetal weight derived from these equations is then compared to distributions normalized for gestational age to identify growth outside the norm. Since abnormalities of fetal growth are associated with an increased risk of adverse outcome, this information often affects how the pregnancy and delivery will be managed.
Techniques for obtaining biometric measurements used in assessment of gestational age and fetal weight can be found separately. (See "Prenatal assessment of gestational age and estimated date of delivery".)
Ultrasound examination typically involves measurement of multiple biometric parameters that are incorporated into a formula for calculating estimated fetal weight (EFW). Most commonly, a combination of biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC), and femur length (FL) is used. The two most popular formulas are Warsof's  with Shepard's modification  and Hadlock's [3,4]. These formulas are included in most ultrasound equipment packages. However, at least 30 formulas for estimating fetal weight have been published.
Performance — A well-designed prospective study compared estimated and actual birth weight in 441 pregnancies delivered within 48 hours of ultrasonography . Twenty-nine different formulas were used to calculate EFW. All of the pregnancies were singletons and 99 percent of the mothers were Caucasian. For birthweights between 3.0 and 3.5 kg, 80 percent of EFWs were within 10 percent of birthweight. Formulas using head, abdomen, and femur measurements resulted in the lowest mean absolute percentage error (about 8 percent). A retrospective cohort study of the accuracy of estimated fetal weight within one week of delivery using 23 fetal weight estimation models concluded the Sabbagha formula , which like Hadlock uses head, abdomen, and femur measurements but also incorporates gestational age, resulted in the lowest mean percentage error (-1.5 to 3.3 percent) .
A systematic review of studies that compared ultrasound estimated fetal weight (EFW) with birth weight included 11 different methods of fetal weight assessment . These studies consistently observed that in 5 percent of fetuses, the random error in fetal weight estimation exceeded 14 percent of birth weight. Both intraobserver and interobserver variability was large. The authors concluded that volumetric methods had some advantages, but there was no consistently superior method of sonographic determination of fetal weight.