CRL-Based Gestational Age Calculator
Estimate gestational age and estimated due date (EDD) from Crown-Rump Length (CRL) using the Robinson & Fleming (1975) formula — the international clinical standard.
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EDD = Scan Date + (280 − GA days)
What is Crown-Rump Length (CRL)?
Crown-Rump Length is the measurement of the embryo or fetus from the top of the head (crown) to the bottom of the buttocks (rump). It is the most accurate parameter for dating pregnancy in the first trimester.
CRL is typically measured between 6 and 14 weeks of gestation and is considered more reliable than last menstrual period (LMP) dating when performed correctly.
How is Gestational Age Calculated from CRL?
This calculator uses the classic Robinson & Fleming (1975) formula, which remains the international clinical standard adopted by NICE, RCOG, ISUOG and most ultrasound machines:
GA (days) = 8.052 × √CRL (mm) + 23.73
The result is expressed as completed weeks and days. The estimated due date is then calculated by adding the remaining days to reach 280 days (40 weeks) from the scan date.
CRL → Gestational Age Reference Table
| CRL (mm) | Gestational Age | Approx. Size |
|---|---|---|
| 5 | 6w 0d | Grain of rice |
| 10 | 7w 1d | Blueberry |
| 15 | 7w 6d | Kidney bean |
| 20 | 8w 4d | Grape |
| 25 | 9w 2d | Strawberry |
| 30 | 9w 6d | Cherry |
| 35 | 10w 2d | Fig |
| 40 | 10w 6d | Lime |
| 45 | 11w 2d | Plum |
| 50 | 11w 5d | Lemon |
| 55 | 12w 1d | Lime |
| 60 | 12w 3d | Pea pod |
| 65 | 12w 6d | Passion fruit |
| 70 | 13w 1d | Peach |
| 75 | 13w 4d | Nectarine |
| 80 | 13w 6d | Lemon |
| 84 | 14w 0d | Apple |
Clinical Notes
- CRL is the most accurate method for dating pregnancy in the first trimester (accuracy ±3–5 days between 7–13 weeks).
- Most reliable range: CRL 5–84 mm (approximately 6 weeks 2 days to 14 weeks 0 days).
- After CRL > 84 mm (around 14 weeks), dating should switch to biparietal diameter (BPD) and head circumference.
- When LMP and CRL differ beyond established thresholds (usually >5–7 days), the ultrasound date is generally preferred.
CRL Gestational Age Calculator – Frequently Asked Questions
What formula does this calculator use?
The Robinson & Fleming (1975) formula: GA (days) = 8.052 × √CRL (mm) + 23.73. This is the international clinical standard.
What is the valid CRL range?
The formula is most accurate between 5 mm and 84 mm (approximately 6 weeks 2 days to 14 weeks 0 days).
How is the estimated due date calculated?
EDD = Scan date + (280 − gestational age in days). If no scan date is entered, today’s date is used.
Is CRL more accurate than LMP dating?
Yes. When performed correctly in the first trimester, CRL is generally more accurate than LMP-based dating, especially when menstrual history is uncertain.
What if CRL is outside 5–84 mm?
Results outside this range are approximate. Below 5 mm or above 84 mm, other measurements or formulas may be more appropriate.
Can this replace the ultrasound report?
No. This tool is for educational and reference purposes. Always follow the official ultrasound report and clinical judgment.
Medical Disclaimer: This calculator is intended for educational and clinical reference purposes. Gestational age and estimated due date calculated from CRL are estimates based on the Robinson & Fleming formula and depend on accurate measurement technique. This calculator should not replace professional obstetric assessment, the official ultrasound report, or individualized medical advice.
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