gestational age CRL first trimester ultrasound
CRL, measured by ultrasound between 6 and 14 weeks, feeds the INTERGROWTH-21st formula this calculator uses: gestational age in days = 40.9041 + 3.21585 × √CRL + 0.348956 × CRL, with CRL in millimeters. This page shows how that formula converts a CRL value into weeks and why its margin of error is the smallest among all the biometric measurements the tool accepts.
It is the smallest value the calculator accepts for this measurement type, corresponding to the start of the CRL usage window.
In this range, the 1st-trimester screening exam (nuchal translucency) is commonly performed.
Above this value, the calculator no longer accepts CRL as a measurement, dating then depends on BPD, HC, AC or FL.
CRL (Crown-Rump Length) in the 1st trimester (weeks 6–14) is the most precise parameter, with a margin of error of only ± 3–5 days. After week 14, composite biometry (BPD + HC + AC + FL) tends to be more accurate than any single measurement. Precision decreases progressively throughout gestation.
They are the same measurement with different names by language: CCN is comprimento cabeça-nádega, in Portuguese, CRL is crown-rump length, in English. Both terms feed the same INTERGROWTH-21st formula, with no additional conversion between them.
Because above that value the embryo has grown past the range the INTERGROWTH-21st formula was derived and validated for, roughly 5 to 84 mm (about 6 to 14 weeks). Outside that range, dating shifts to other biometric measurements. The 5 mm floor exists for the same reason in reverse: below that size the embryo is too small for the formula's derivation data to apply reliably.
No. The calculator applies the same 0.5-week margin to any CRL within the accepted range, without scaling with embryo size. This contrasts with the 2nd- and 3rd-trimester measurements, whose margin grows along with gestational age.
Medical disclaimer: this calculation is educational and does not replace physician evaluation. Biometric measurements have inherent margin of error and must always be interpreted in clinical context.
Select the biometric measurement type from your ultrasound report, enter the value in millimeters, and optionally enter the exam date to get the estimated EDD.
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