Estimate fetal weight using the Hadlock formula or gestational age percentile lookup. Track your baby's growth throughout pregnancy.
Fetal weight estimation provides crucial insight into your baby's growth and health during pregnancy. Healthcare providers use ultrasound measurements and standardized formulas like the Hadlock formula to calculate estimated fetal weight (EFW) at each prenatal visit from the second trimester onward.
This calculator offers two approaches: enter your gestational age to see the average fetal weight for that week based on published growth charts, or enter specific ultrasound measurements (if available) for a more personalized estimate. The reference data comes from widely used growth percentile charts.
Fetal weight is one of the key indicators providers use to assess whether a baby is growing appropriately. Weights below the 10th percentile or above the 90th percentile may prompt additional monitoring or changes in the delivery plan. Whether you are a beginner or experienced professional, this free online tool provides instant, reliable results without manual computation. By automating the calculation, you save time and reduce the risk of costly errors in your planning and decision-making process.
Tracking estimated fetal weight helps identify potential growth concerns early. Small-for-gestational-age (SGA) babies may indicate placental insufficiency, while large-for-gestational-age (LGA) babies may suggest gestational diabetes. Both conditions benefit from early detection and management. This calculator helps you understand your baby's size relative to averages. Having a precise figure at your fingertips empowers better planning and more confident decisions.
Hadlock Formula (simplified): log₁₀(EFW) = 1.304 + 0.05281×AC + 0.01938×FL − 0.004×AC×FL Where: EFW = Estimated Fetal Weight (grams) AC = Abdominal Circumference (cm) FL = Femur Length (cm) GA-based lookup uses published 50th percentile charts.
Result: ~1,319 g (2 lb 15 oz)
At 30 weeks gestational age, the average fetal weight is approximately 1,319 grams (about 2 pounds 15 ounces). The 10th percentile is around 1,100 g and the 90th percentile is around 1,550 g. The baby is gaining about 200 grams per week at this stage.
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.
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.
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.
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.
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.
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.
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.
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.
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.