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Estimates gestational age from an ultrasound crown-rump length (CRL) measurement using the standard published Robinson & Fleming regression formula (Robinson HP, Fleming JE, "A critical evaluation of sonar crown rump length measurements", British Journal of Obstetrics and Gynaecology, 1975): GA (days) = 8.052 x sqrt(CRL in mm) + 23.73. This is a widely-used first-trimester dating estimate, most accurate between roughly 6 and 13 weeks. For general reference and educational purposes only — an estimate to discuss with a healthcare provider, not a replacement for clinical ultrasound dating.
How it works
Enter the crown-rump length measurement in millimeters from an ultrasound report. The tool applies the Robinson & Fleming formula and converts the result to weeks and days.
- Enter crown-rump length (mm).
- Click Calculate to see your results.
Examples
CRL 20 mm
GA = 8.052 x sqrt(20) + 23.73 = 8.052 x 4.472 + 23.73 ≈ 60.75 days ≈ 8 weeks, 5 days.
Who should use it
- Reviewing an early ultrasound report alongside a healthcare provider.
- Obstetric education about first-trimester dating.
Industry applications
- Obstetric ultrasound reference
- Maternal-fetal medicine education
Advantages
- Standard, widely published, long-established formula.
- One of the more accurate first-trimester dating methods.
Limitations
- Only valid in the first trimester, before the fetus is too large for CRL measurement.
- An estimate — actual due dates are set by your care provider using the full clinical picture.
Common mistakes to avoid
- Using a CRL measurement taken outside the roughly 6-13 week window where the formula is validated.
- Confusing crown-rump length with other measurements like biparietal diameter.
Best practices
- Use the CRL value directly from your ultrasound report for the most accurate result.
Tips
- If you also have a last-menstrual-period-based due date, your provider will typically use CRL dating instead if the two differ significantly.