CRL-Based Gestational Age Calculator
Calculate gestational age and estimated due date from Crown-Rump Length (CRL) using the Robinson & Fleming (1975) formula, the international clinical standard for first-trimester pregnancy dating.
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Estimated Due Date = Scan Date + (280 − Gestational Age in 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 obtained on ultrasound in the mid-sagittal plane and represents the longest straight-line measurement of the embryo or early fetus.
CRL is the preferred biometric parameter for estimating gestational age in the first trimester because embryonic and early fetal growth is relatively uniform during this period.
How is Gestational Age Calculated from CRL?
When a reliable CRL measurement is available, gestational age can be calculated using established regression formulae. This calculator applies the classic Robinson & Fleming (1975) equation:
Gestational Age (days) = 8.052 × √CRL (mm) + 23.73
The result is expressed in completed weeks and remaining days. The estimated due date is derived by adding the number of days required to reach a conventional pregnancy duration of 280 days from the date of the ultrasound examination.
The Robinson & Fleming formula remains the international clinical standard and is widely incorporated into ultrasound equipment and professional guidelines.
Clinical Accuracy and Valid Range
CRL-based dating is most accurate between approximately 6 weeks and 14 weeks of gestation. Within the validated range of 5 mm to 84 mm, the formula provides gestational age estimates with an accuracy of approximately ±3 to ±5 days.
Outside this range the reliability of the formula declines. Measurements below 5 mm or above 84 mm should be interpreted with caution, and alternative biometric parameters may be more appropriate.
CRL and Corresponding Gestational Age
| CRL (mm) | Gestational Age | Approximate Clinical Stage |
|---|---|---|
| 5 | 6 weeks 0 days | Early embryonic period |
| 10 | 7 weeks 1 day | Early first trimester |
| 15 | 7 weeks 6 days | Early first trimester |
| 20 | 8 weeks 4 days | First trimester |
| 25 | 9 weeks 2 days | First trimester |
| 30 | 9 weeks 6 days | First trimester |
| 35 | 10 weeks 2 days | First trimester |
| 40 | 10 weeks 6 days | First trimester |
| 45 | 11 weeks 2 days | Late first trimester |
| 50 | 11 weeks 5 days | Late first trimester |
| 55 | 12 weeks 1 day | Late first trimester |
| 60 | 12 weeks 3 days | Late first trimester |
| 65 | 12 weeks 6 days | Late first trimester |
| 70 | 13 weeks 1 day | Transition to second trimester |
| 75 | 13 weeks 4 days | Transition to second trimester |
| 80 | 13 weeks 6 days | Transition to second trimester |
| 84 | 14 weeks 0 days | Early second trimester |
Gestational Age and Pregnancy Trimesters
| Trimester | Gestational Age Range | Clinical Context |
|---|---|---|
| First Trimester | 0 weeks to 13 weeks 6 days | Period of highest accuracy for CRL dating |
| Second Trimester | 14 weeks to 27 weeks 6 days | CRL becomes less reliable; alternative biometry preferred |
| Third Trimester | 28 weeks onward | CRL is not used for dating |
Why is CRL-Based Dating Important?
Accurate assignment of gestational age is fundamental to obstetric care. First-trimester CRL measurement provides one of the most reliable methods for establishing pregnancy dating when performed correctly.
- Establishing the expected timing of pregnancy milestones
- Interpreting subsequent ultrasound findings
- Planning prenatal screening and diagnostic tests
- Monitoring fetal growth trajectories
- Assessing pregnancy progression
- Supporting clinical decision-making when LMP is uncertain
CRL Dating Compared with LMP Dating
Last menstrual period dating remains useful when the first day of the LMP is known with certainty and menstrual cycles are regular. However, when the LMP is uncertain, cycles are irregular, or there is a clinically significant discrepancy between menstrual dating and ultrasound measurements, CRL-based dating is generally preferred in the first trimester.
Professional guidelines commonly recommend using the earliest reliable ultrasound measurement, particularly CRL, to establish or revise the estimated due date when the difference exceeds established thresholds.
Factors That Can Affect CRL-Based Dating
| Factor | Potential Effect |
|---|---|
| Measurement technique | Incorrect plane or inclusion of limbs may alter the result. |
| CRL outside validated range | Accuracy declines below 5 mm or above 84 mm. |
| Fetal position or maternal factors | May reduce image quality and measurement precision. |
| Multiple pregnancy | Dating should be based on the larger or more advanced fetus according to local protocol. |
| Late first-trimester measurement | Biological variation increases as gestation advances. |
CRL-Based Gestational Age Calculator – Frequently Asked Questions
What formula does this calculator use?
The Robinson & Fleming (1975) formula: Gestational Age (days) = 8.052 × √CRL (mm) + 23.73. This remains the international clinical standard.
What is the recommended CRL range?
The formula is most accurate between 5 mm and 84 mm, corresponding approximately to 6 weeks 2 days through 14 weeks 0 days.
How is the estimated due date derived?
The estimated due date is calculated by adding the remaining days required to reach 280 days of gestation from the date of the ultrasound examination.
Is CRL more accurate than LMP dating?
When performed correctly in the first trimester, CRL is generally more accurate than LMP-based dating, particularly when the menstrual history is uncertain or cycles are irregular.
What if the CRL is outside the validated range?
Results outside 5–84 mm should be interpreted with caution. Alternative biometric parameters are preferred once the measurement exceeds the upper limit of the formula.
Can this calculator replace the ultrasound report?
No. This tool is intended for educational and clinical reference purposes. Clinical decisions should be based on the official ultrasound report and professional judgment.
Medical Disclaimer: This calculator is intended for educational and clinical reference purposes. Gestational age and estimated due date derived from Crown-Rump Length are estimates based on the Robinson & Fleming formula and depend on accurate ultrasound measurement technique and appropriate clinical context. This calculator should not replace professional obstetric assessment, the official ultrasound report, or individualized medical advice.
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