gestational age CRL first trimester ultrasound

Gestational Age from CRL in the 1st Trimester

CRL, measured by transvaginal ultrasound between 6 and 14 weeks, is the most precise gestational dating parameter available in clinical practice, with a margin of error of only 3–5 days.

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.

Biometric measurement type

Crown-Rump Length (CRL)in mmUltrasound exam dateOptional — to calculate EDD

How to use this calculator

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.

CRL1st trimester · 6–14 wks

BPD2nd/3rd trim. · 12–40 wks

HC2nd/3rd trim. · 12–40 wks

AC2nd/3rd trim. · 14–40 wks

FL2nd/3rd trim. · 14–40 wks

The margin of error for biometry-based estimates is ± 0.5 to 2.5 weeks, depending on the measurement and gestational age. CRL in the 1st trimester is the most precise parameter.

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Why is CRL so accurate?

In the first trimester, all human embryos grow at a highly predictable rate, almost independent of genetic and maternal nutritional factors. This creates a near-linear relationship between CRL in mm and gestational age in days.
The INTERGROWTH-21st study, conducted in 10 countries with different nutritional and ethnic profiles, confirmed that CRL growth curves are broadly universal, making the derived formula applicable globally.

CRL-based estimation examples

CRL of 10 mm

Input: CCN: 10 mm
Expected output: ~7 semanas e 1 dia

Margin of error: ± 3 days at this stage.

CRL of 45 mm

Input: CCN: 45 mm
Expected output: ~11 semanas e 2 dias

First-trimester screening (NT + PAPP-A) can be performed at this stage.

Full tool FAQ

Which measurement is most accurate for estimating GA?

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.

Can ultrasound 'correct' the due date calculated from LMP?

Why does the margin of error increase later in pregnancy?

Can this calculator be used for twin pregnancies?

What is the clinical significance of each measurement?

Which formulas does the calculator use?

Can I combine more than one measurement for better accuracy?

What should I do if the GA estimated here differs from my ultrasound report?

Frequently asked questions

What is the difference between CCN and CRL?

They are the same measurement with different names by language. CCN = comprimento cabeça-nádega (Portuguese). CRL = crown-rump length (English). Both refer to the distance from the top of the head to the coccyx of the embryo/fetus.