Calculator
Gestational Age to CRL
Enter a gestational age to find the expected crown-rump length (CRL) range using the Robinson & Fleming (1975) formula — the international clinical standard for first-trimester dating.
Gestational Age to CRL
Reverse Calculator—
Expected Crown-Rump Length
CRL Growth Chart
About This Calculator
Method
This reverse calculator inverts the validated CRL-to-GA formulas to predict the expected crown-rump length for a given gestational age. The 95% confidence interval is derived by computing the CRL at the gestational age ± the formula’s known accuracy (±3–5 days depending on the CRL range).
Valid Range
Gestational ages between approximately 6 weeks 0 days and 14 weeks 1 day correspond to the validated CRL range of 5–84 mm. Outside this window, accuracy decreases and alternative dating methods (e.g., HC or BPD) should be considered.
Accuracy
| CRL Range | 95% CI |
|---|---|
| < 5 mm | Not reliable |
| 5–10 mm | ±4 days |
| 11–50 mm | ±3 days |
| 51–84 mm | ±5 days |
| > 84 mm | Not applicable |
Formulas
Robinson & Fleming (1975) — GA (days) = 8.052 × √CRL + 23.73
Westerway et al. (2000) — GA (weeks) = −0.0007 × CRL² + 0.1584 × CRL + 5.2876
P. Doubilet — GA (weeks) = CRL × 0.1 + 5.7
Clinical Use
This tool is useful when planning ultrasound timing, estimating expected CRL at a known gestational age (e.g., from LMP or IVF dating), and for quality assurance when the measured CRL deviates significantly from the expected range.
Limitations
- The inverse formulas assume a normal fetal growth trajectory and may not apply to growth-restricted or anomalous fetuses.
- CRL variability increases with gestational age; the reported range is a statistical estimate, not an absolute boundary.
- Maternal obesity, fibroids, and multiple gestation can affect measurement accuracy.
- Population-specific variations exist; these formulas are derived from European/North American cohorts.