Fetal Weight Estimator

About the Fetal Weight Estimator

Estimated fetal weight (EFW) is one of the numbers providers use to judge whether growth looks broadly on track for a given gestational age. It is usually based on ultrasound measurements and formulas such as the Hadlock method, then interpreted against percentile charts.

This calculator lets you look up an average fetal weight by gestational week or compare your ultrasound estimate with a typical range. That can help you understand what a report means when it says a baby is around the 20th, 50th, or 90th percentile.

It is still an estimate, not a scale weight. The real value is usually in the growth trend over time and in the broader clinical picture, not in chasing a single number.

When This Page Helps

This page is most useful when you want to interpret an ultrasound estimate in plain language. It can help you understand whether a reported weight looks average for the week, what SGA or LGA means in context, and why providers care more about repeated growth patterns than one isolated scan.

How to Use the Inputs

1. Enter your current gestational age in weeks.
2. View the average fetal weight for that week and the typical range (10th-90th percentile).
3. Optionally compare with the EFW from your most recent ultrasound.
4. Track weight estimates over multiple visits to observe your baby's growth trajectory.
5. Discuss any concerns about growth with your healthcare provider.

Example Calculation

Tips & Best Practices

• Ultrasound weight estimates have a margin of error of 10-15% — a 3,000 g estimate could be 2,550-3,450 g.
• Fetal weight gain accelerates in the third trimester — about 200-250 g per week from weeks 28-36.
• Consistent growth along a percentile curve is more important than the specific percentile.
• Gestational diabetes screening at 24-28 weeks helps catch conditions that affect fetal weight.
• If your baby's EFW is below the 10th percentile, additional Doppler studies may be recommended.
• Average full-term birth weight is about 3,400 g (7 lb 8 oz), with a wide normal range of 2,500-4,000 g.

Fetal Weight by Gestational Age

Fetal weight increases exponentially during pregnancy. At 12 weeks, the fetus weighs about 14 grams. By 20 weeks, it reaches approximately 300 grams. At 30 weeks, the average is about 1,300 grams, and by 40 weeks, the typical weight is 3,400 grams. The most rapid weight gain occurs between 28 and 36 weeks.

Understanding Growth Percentiles

Growth percentiles compare your baby's weight to thousands of other babies at the same gestational age. The 50th percentile is average. Being at the 25th percentile means 75% of babies are heavier and 25% are lighter. What matters most is consistent growth along a similar percentile over time.

When to Be Concerned

A single measurement is less important than the trend. If your baby drops from the 50th to the 10th percentile between scans, your provider may order additional testing such as Doppler ultrasound to assess blood flow. Rising weight that crosses above the 90th percentile may prompt glucose screening.

Frequently Asked Questions

• How accurate are fetal weight estimates?

  Ultrasound-based fetal weight estimates are accurate to within ±10-15%. A baby estimated at 3,000 g could actually weigh between 2,550 g and 3,450 g. Accuracy decreases as the baby gets larger in the third trimester.

• What is small for gestational age (SGA)?

  SGA means the estimated fetal weight is below the 10th percentile for gestational age. It may indicate intrauterine growth restriction (IUGR), which can be caused by placental issues, maternal health conditions, or genetic factors.

• What is large for gestational age (LGA)?

  LGA means the estimated weight is above the 90th percentile. Common causes include gestational diabetes, maternal obesity, and genetic factors. LGA babies may have a higher risk of delivery complications.

• How often is fetal weight estimated?

  A standard anatomy scan at 18-22 weeks includes weight estimation. Additional growth scans may be done at 28-32 weeks and 36 weeks, especially for high-risk pregnancies. Serial scans track growth over time.

• Can I increase my baby's weight?

  Fetal weight is primarily determined by genetics and placental function. Adequate maternal nutrition, prenatal vitamins, and managing conditions like hypertension can support healthy growth, but there is no way to directly control fetal weight.

• Why do weight estimates differ between ultrasound machines?

  Different machines, sonographers, and formulas can produce slightly different estimates. This is why tracking trends over time using the same facility is more informative than comparing single measurements from different sources.