Fetal Weight Estimator

Understanding Fetal Growth & Weight Estimation

What Is Fetal Weight Estimation?

Fetal weight estimation is the process of estimating the weight of a baby while still in the womb. It is typically done using ultrasound measurements of the baby's head, abdomen, and limbs. The most commonly used formula is the Hadlock formula, which combines multiple measurements to provide an estimated fetal weight (EFW).

This estimator uses standard growth chart data from large population studies to provide an average weight and length for each week of pregnancy, along with percentile ranges to help you understand your baby's growth pattern.

The Hadlock Formula

The Hadlock formula, developed by Dr. Frank Hadlock in 1985, is the most widely used method for estimating fetal weight:

Log₁₀(EFW) = 1.3596 − 0.00386(AC×FL) + 0.0064(HC) + 0.00061(BPD×AC) + 0.0424(AC) + 0.174(FL)

Where:

• BPD: Biparietal diameter (head width)
• HC: Head circumference
• AC: Abdominal circumference
• FL: Femur length

This estimator uses the Hadlock growth chart data to provide weight estimates based on gestational age alone, without requiring ultrasound measurements.

Growth Chart Standards

• Hadlock (1985): The most widely used standard in clinical practice. Based on a US population from the 1980s. Provides weight estimates for 12-40 weeks.
• INTERGROWTH-21st (2014): An international standard based on data from healthy pregnancies in Brazil, China, India, Italy, Kenya, Oman, UK, and USA. Provides percentiles for 14-40 weeks.
• WHO (2006): World Health Organization growth standards based on a multinational study. Often used for newborn and infant growth assessment.

What Do Percentiles Mean?

Percentiles show how your baby's estimated weight compares to other babies of the same gestational age:

• Above 90th percentile: Large for gestational age (LGA) — may indicate macrosomia.
• 50th percentile: Average weight — about half of babies are smaller and half are larger.
• Between 10th and 90th: Normal range.
• Below 10th percentile: Small for gestational age (SGA) — may indicate growth restriction.

Factors Affecting Fetal Growth

• Maternal nutrition: Adequate protein, calories, and nutrients support optimal growth.
• Genetics: Parental height and weight influence fetal growth potential.
• Placental function: The placenta provides nutrients and oxygen to the baby.
• Maternal health: Conditions like diabetes, hypertension, and preeclampsia can affect growth.
• Multiple pregnancy: Twins and triplets typically grow slower than singletons.
• Smoking and alcohol: Both are associated with reduced fetal growth.
• Ethnicity: Growth patterns may vary among different ethnic groups.

When to Be Concerned

• Consistently below the 10th percentile: May indicate fetal growth restriction (FGR) — requires monitoring.
• Consistently above the 90th percentile: May indicate fetal macrosomia — associated with delivery complications.
• Sudden change in growth trajectory: A significant drop or increase in percentiles warrants investigation.
• Reduced fetal movements: Always contact your healthcare provider immediately.

References & Further Reading

• Hadlock, F.P., Harrist, R.B., & Martinez-Poyer, J. (1991). In utero analysis of fetal growth: a sonographic weight standard. Radiology, 181(1), 129-133.
• Hadlock, F.P. et al. (1985). Estimation of fetal weight with the use of head, body, and femur measurements. American Journal of Obstetrics & Gynecology, 151(3), 333-337.
• INTERGROWTH-21st Consortium. (2014). International standards for fetal growth based on serial ultrasound measurements. The Lancet, 384(9946), 869-879.
• Papageorghiou, A.T. et al. (2014). International standards for fetal growth: the INTERGROWTH-21st project. American Journal of Obstetrics & Gynecology, 210(3), 239-244.
• American College of Obstetricians and Gynecologists. (2021). Fetal Growth Restriction. Practice Bulletin No. 227.