What are the typical estimated fetal weights for each gestational age (weeks) in a normal singleton pregnancy?

Estimated Fetal Weight by Gestational Age

For clinical assessment of fetal size, use the Hadlock three-parameter formula (incorporating head circumference, abdominal circumference, and femur length) plotted on population-based reference charts, recognizing that appropriate-for-gestational-age (AGA) fetuses typically fall between the 10th and 90th centiles for their gestational age. 1, 2

Standard Reference Values and Methodology

Recommended Formula for EFW Calculation

• The Hadlock equation using HC, AC, and FL is the most accurate model for estimating fetal weight and is recommended over other formulas, including the original Intergrowth 21 equation 1
• Population-based fetal growth references (such as Hadlock) should be used in determining fetal weight percentiles 2
• The addition of biparietal diameter (BPD) to the Hadlock equation does not increase accuracy 1

Normal Growth Parameters

• An appropriate-for-gestational-age (AGA) fetus has measurements between the 10th and 90th centiles 1
• Small-for-gestational-age (SGA) is defined as EFW or AC below the 10th centile 1, 2
• Severe fetal growth restriction is defined as EFW less than the 3rd percentile 2

Growth Patterns Across Gestation

General Growth Trajectory

• Fetal weight shows continuous growth until term with a linear correlation between EFW and advancing gestational age 3
• Maximum gain in EFW (34.05%) occurs after the second trimester (after 28 weeks) 3
• The distribution of EFW is asymmetric in the population: wider variation below the 50th percentile in early second trimester, reversing in third trimester with wider variation above the 50th percentile 4

Specific Weight Ranges

• At 37 weeks gestation, the physiologic range (5th-95th percentile) for EFW spans from 2205-3538 grams 4
• At 38 weeks, maximum and minimum fetal weights range from approximately 2567 to 3389 grams 3
• Median gestational age at birth is 39 weeks with median birthweight of 3300 grams 4

Factors Affecting Fetal Weight

Fetal Factors

• Male fetuses are calculated to be 5% heavier than female fetuses at 20 weeks and 3% heavier at 40 weeks 5
• Breech presentation is associated with a 1.3% weight difference compared to cephalic presentation 5

Maternal Factors

• Each increase in maternal height tertile corresponds to a 2.5% increase in EFW 5
• Maternal age >34 years is associated with 4.5% increased EFW compared to age <24 years 5
• Smoking is associated with a 4% reduction in EFW 5
• Maternal height, weight, ethnic group, parity, and smoking all have significant independent effects on birth weight (1-4.5% each) 4

Important Clinical Considerations

Measurement Precision and Quality

• Recognize that EFW has inherent imprecision of ±10%, with greater error at extremes of fetal weight 1
• All measurements should follow the same methodology as used in the studies that produced the reference curves being applied 1
• Quality control of images and measurements should be routine practice 1

Population Variations

• Ethnic, cultural, and geographic variations play a significant role in fetal weight, even after adjusting for maternal covariates 4
• Population-specific reference charts may be needed as EFW can be at lower ranges in certain populations (e.g., Indian population) compared to Western references 3
• The wide physiologic variation in human fetal growth under optimized conditions is part of normal biologic diversity 4

Pitfalls to Avoid

• Changing formulas during growth monitoring can lead to artifactual changes in growth trajectory and must be accounted for when comparing serial assessments 1
• Preterm infants have lower birthweight distribution compared to expected EFW of ongoing pregnancies at the same gestational age, with the 5th centile for birthweight being 30% lower than EFW at 27 weeks 6
• Abdominal circumference alone is comparable to EFW in predicting SGA, but EFW combining three measurements (HC, AC, FL) provides superior prediction 1