Fetal Weight Estimator
Estimate baby weight and length by gestational week.
Last updated: June 13, 2026
Calculator
Results
Week 24
Crown to heel
How to Use This Calculator
- 1Enter your current gestational week (8-42).
- 2View estimated fetal weight in grams/kg and ounces/pounds.
- 3See estimated length in centimeters and inches.
- 4Track whether measurement is crown-to-rump or crown-to-heel.
Formula Explanation
Fetal weight and length are estimated from population averages at each gestational week. Before week 20, length is measured crown-to-rump (head to bottom). After week 20, it is measured crown-to-heel (head to foot). These are 50th percentile estimates.
Variables:
Worked Examples
Week 12
- 1. Weight: ~14 grams (0.5 oz)
- 2. Length: 5.4 cm (2.1 in) crown-to-rump
- 3. Trimester: 1st
- 4. All major organs formed
Week 32
- 1. Weight: ~1,702 grams (3.8 lbs)
- 2. Length: 42.4 cm (16.7 in) crown-to-heel
- 3. Trimester: 3rd
- 4. Major weight gain phase
Why Use This Calculator
- Week-by-week weight and length estimates from 8 to 42 weeks.
- Converts between metric and imperial units automatically.
- Shows whether measurement is crown-to-rump or crown-to-heel.
- Includes growth phase descriptions for context.
Tips & Best Practices
- These are 50th percentile averages — your baby may be smaller or larger and still be perfectly healthy.
- Ultrasound weight estimates have a margin of error of ±15% in the third trimester.
- Babies gain the most weight in weeks 28-40, averaging 200-250 grams per week.
- Fetal length measurements switch from crown-to-rump to crown-to-heel at 20 weeks.
Common Mistakes to Avoid
❌ Worrying because your baby measures differently from the chart.
✅ Growth charts show averages. Your provider tracks YOUR baby's growth curve over time. Consistent growth along any percentile is healthy.
❌ Thinking ultrasound weight estimates are exact.
✅ Ultrasound estimates can be off by 10-15% or more, especially in the third trimester. A baby estimated at 7 lbs could actually weigh 6-8 lbs.
Frequently Asked Questions
How much does a baby weigh at 20 weeks?
When do babies gain the most weight?
What is the average birth weight?
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<iframe src="https://redicalc.com/embed/health/fetal-weight-calculator" width="100%" height="600" style="border:none;border-radius:12px;box-shadow:0 1px 3px rgba(0,0,0,0.1);" title="Fetal Weight Estimator — RediCalc.com" loading="lazy" allowtransparency="true"></iframe>Disclaimer: This calculator is provided for informational and educational purposes only. Results are estimates and should not be used as a substitute for professional advice. RediCalc.com makes no guarantees regarding the accuracy, completeness, or suitability of any calculations. Always consult a qualified professional before making financial, health, construction, or other important decisions.
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