What are the guidelines for assessing and managing Estimated Fetal Weight (EFW)?

Guidelines for Assessing and Managing Estimated Fetal Weight (EFW)

Estimated fetal weight (EFW) should be determined using ultrasound measurements of fetal biometry, with fetal growth restriction (FGR) defined as an EFW or abdominal circumference below the 10th percentile for gestational age. 1

Diagnostic Criteria and Assessment

Definition and Measurement

• Fetal growth restriction (FGR) is defined as an ultrasonographic EFW or abdominal circumference (AC) below the 10th percentile for gestational age 1
• Population-based fetal growth references (such as Hadlock) should be used in determining fetal weight percentiles 1
• Accurate pregnancy dating is an essential prerequisite for diagnosing FGR, with first-trimester crown-rump length being the most reliable method 1
• Ultrasonographic EFW is generated using regression equations that combine biometric measurements of fetal biparietal diameter, head circumference, abdominal circumference, and femur length 1

Accuracy Considerations

• Formulas based on 3 or 4 fetal biometric indices are significantly more accurate in estimating fetal weights than formulas based on 1 or 2 indices 1
• Accuracy of EFW can be improved through averaging of multiple measurements, improvements in image quality, uniform calibration of equipment, and regular audits 1
• Most EFW formulas tend to overestimate weight in low birthweight babies and underestimate weight in babies >3500g 2
• Ultrasound EFW is more accurate than clinical estimation for predicting low birth weight fetuses (<3000g) 3

Management Protocol Based on EFW Findings

Initial Evaluation

• When FGR is diagnosed, a detailed obstetrical ultrasound examination (CPT code 76811) should be performed, particularly with early-onset FGR (<32 weeks of gestation) 1
• Fetal diagnostic testing, including chromosomal microarray analysis, should be offered when FGR is detected with fetal malformation, polyhydramnios, or both, regardless of gestational age 1
• Prenatal diagnostic testing with chromosomal microarray analysis should be offered when unexplained isolated FGR is diagnosed at <32 weeks of gestation 1

Surveillance Protocol

• Once FGR is diagnosed, serial umbilical artery Doppler assessment should be performed to assess for deterioration 1
• For pregnancies with decreased end-diastolic velocity (flow ratios >95th percentile) or severe FGR (EFW <3rd percentile), weekly umbilical artery Doppler evaluation is recommended 1
• When umbilical artery absent end-diastolic velocity (AEDV) is detected, Doppler assessment should be performed 2-3 times per week 1
• In the setting of reversed end-diastolic velocity (REDV), hospitalization, administration of antenatal corticosteroids, heightened surveillance with cardiotocography (CTG) at least 1-2 times per day, and consideration of delivery are recommended 1

Timing of Delivery Based on EFW and Doppler Findings

• For pregnancies with FGR and normal umbilical artery Doppler when EFW is between 3rd and 10th percentile, delivery at 38-39 weeks of gestation is suggested 1
• For pregnancies with FGR and decreased diastolic flow (without AEDV/REDV) or with severe FGR (EFW <3rd percentile), delivery at 37 weeks of gestation is recommended 1
• For pregnancies with FGR and AEDV, delivery at 33-34 weeks of gestation is recommended 1
• For pregnancies with FGR and REDV, delivery at 30-32 weeks of gestation is recommended 1

Classification of FGR

Early vs. Late Onset FGR

• Early-onset FGR is diagnosed before 32 weeks of gestation, while late-onset FGR is diagnosed at or after 32 weeks 1
• Early-onset FGR is typically more severe, follows an established Doppler pattern of fetal deterioration, is more commonly associated with maternal hypertensive disorders, and shows more significant placental dysfunction 1
• Late-onset FGR represents approximately 70%-80% of FGR cases, is typically milder, less likely to be associated with maternal hypertensive disorders, and has less extensive placental histopathologic findings 1

Severity Assessment

• An EFW below the 3rd percentile is associated with increased risk of adverse perinatal outcome regardless of umbilical and middle cerebral artery Doppler indices 1
• The presence of abnormal umbilical artery Doppler indices predicts adverse perinatal outcomes 1
• Maternal hypertension is an important independent determinant of poor outcomes in FGR pregnancies 1

Common Pitfalls and Caveats

• All EFW formulas either under or overestimate birthweight in singleton pregnancies, with most overestimating weight in low birthweight babies and underestimating weight in babies >3500g 2, 4
• Accuracy of EFW decreases with increasing maternal BMI, particularly for term fetuses 5
• Sildenafil and activity restriction are not recommended for in utero treatment of FGR 1
• Low-molecular-weight heparin is not recommended for the sole indication of prevention of recurrent FGR 1
• Screening for toxoplasmosis, rubella, or herpes in pregnancies with FGR is not recommended in the absence of other risk factors 1