terican

Last verified · v1.0

Calculator · health

Crown Rump Length (Crl) Gestational Age Calculator

Calculate gestational age from a fetal CRL measurement using the Robinson-Fleming formula. Enter CRL in millimeters to get estimated gestational age in weeks and days.

FreeInstantNo signupOpen source

Inputs

Estimated Gestational Age

—

Explain my result

0/3 free

Get a plain-English breakdown of your result with practical next steps.

Estimated Gestational Age—weeks

The formula

How the
result is
computed.

How the Crown Rump Length Calculator Works

The Crown Rump Length (CRL) gestational age calculator converts an ultrasound measurement of fetal length into an estimated gestational age. Accurate gestational age estimation is a cornerstone of modern prenatal care, influencing clinical decisions about aneuploidy screening, fetal monitoring, and pregnancy management. The tool applies the Robinson-Fleming formula, first published in 1975 and still the most widely used equation for first-trimester pregnancy dating in clinical obstetrics.

The Robinson-Fleming Formula

The governing equation is:

GA (days) = 8.052 × √CRLmm + 23.73

Where:

  • GA (days) — estimated gestational age in days counted from the last menstrual period (LMP)
  • CRLmm — ultrasound-measured crown-rump length in millimeters
  • 8.052 — empirically derived regression coefficient
  • 23.73 — constant offset in days representing the earliest detectable gestational period

To convert days to weeks and days, divide by 7. For example, 77 days equals 11 weeks and 0 days (77 ÷ 7 = 11 remainder 0).

Derivation and Validation

Robinson and Fleming developed this formula by performing serial ultrasound measurements on pregnancies with precisely known conception dates. A square-root transformation was applied to linearize the embryonic growth curve, which accelerates rapidly in early gestation. The resulting linear regression achieved a high coefficient of determination, confirming that CRL is a strong predictor of gestational age during the first trimester.

The formula has undergone rigorous external validation. The landmark INTERGROWTH-21st multicenter study, published by Papageorghiou AT et al. in Ultrasound in Obstetrics and Gynecology, established international standards for early fetal size and CRL-based pregnancy dating across eight countries on four continents, confirming the formula performs consistently across diverse populations. A further validation study using IVF conception dates — where fertilization timing is precisely documented — confirmed that CRL-derived estimates closely match known embryonic age, with errors typically under five days. That study is available through PubMed Central (PMC3409520).

Variable Definitions and Measurement Protocol

Crown-Rump Length (CRL)

CRL is measured in millimeters from the top of the fetal head (crown) to the lowest point of the fetal buttocks (rump) on a mid-sagittal ultrasound plane. For maximum accuracy:

  • Obtain a true neutral mid-sagittal image with no hyperflexion or hyperextension of the fetal spine
  • Place calipers at the outer edges of the crown and rump
  • Record at least three separate measurements and use the mean value
  • Use transvaginal ultrasound before 10 weeks and transabdominal ultrasound thereafter when image quality allows

Applicable CRL Range

The Robinson-Fleming formula yields reliable results when CRL falls between 10 mm (approximately 6 weeks 3 days) and 84 mm (approximately 13 weeks 6 days). Beyond 84 mm, fetal curling prevents accurate crown-to-rump measurement; clinicians then transition to biparietal diameter (BPD) and head circumference (HC) for ongoing dating.

Worked Examples

Example 1 — CRL = 25 mm

GA = 8.052 × √25 + 23.73 = 8.052 × 5.000 + 23.73 = 40.26 + 23.73 = 63.99 days ≈ 9 weeks 1 day

Example 2 — CRL = 45 mm

GA = 8.052 × √45 + 23.73 = 8.052 × 6.708 + 23.73 = 54.01 + 23.73 = 77.74 days ≈ 11 weeks 1 day

Example 3 — CRL = 65 mm

GA = 8.052 × √65 + 23.73 = 8.052 × 8.062 + 23.73 = 64.93 + 23.73 = 88.66 days ≈ 12 weeks 5 days

Clinical Importance of Accurate Dating

First-trimester CRL measurement is recognized as the gold standard for establishing gestational age in modern obstetric practice. Accurate dating directly affects the timing of aneuploidy screening (nuchal translucency between 11 and 13+6 weeks), the interpretation of maternal serum biochemistry, the scheduling of anomaly scans, and the assessment of fetal growth restriction later in pregnancy. When a discrepancy exists between CRL-derived and LMP-derived gestational ages, clinical guidelines recommend using the ultrasound estimate whenever the difference exceeds five to seven days in the first trimester. Always consult a qualified healthcare provider to interpret ultrasound findings in the context of complete clinical information.

Reference

Frequently asked questions

What is crown rump length (CRL) and how is it measured on ultrasound?
Crown rump length (CRL) is the straight-line distance from the top of the fetal head (crown) to the lowest point of the fetal buttocks (rump), measured in millimeters on a mid-sagittal ultrasound image. Sonographers take at least three separate measurements with the fetus in a neutral position, then average them for accuracy. CRL is the most reliable biometric marker for first-trimester pregnancy dating and is routinely used between 6 and 14 weeks of gestation.
Which formula does the crown rump length calculator use to estimate gestational age?
The calculator applies the Robinson-Fleming formula: GA in days equals 8.052 multiplied by the square root of CRL in millimeters, plus 23.73. This regression equation was derived from serial ultrasound measurements of pregnancies with known conception dates and has been validated by large international studies including the INTERGROWTH-21st project. The result in days converts to weeks and days by dividing by seven.
How accurate is CRL-based gestational age dating compared to other methods?
CRL-based dating in the first trimester is the most accurate method available, with a typical error margin of plus or minus 3 to 5 days when measurements follow standardized technique. Validation research using IVF pregnancies, where conception date is precisely known, confirmed that the Robinson-Fleming formula estimates gestational age within this narrow range. By comparison, last menstrual period dating carries an error of up to two weeks due to variable cycle length and irregular ovulation timing.
At what CRL values is the Robinson-Fleming formula most reliable?
The Robinson-Fleming formula produces the most reliable results when fetal CRL measures between 10 mm (approximately 6 weeks 3 days) and 84 mm (approximately 13 weeks 6 days). Below 10 mm, embryonic structures are too small for consistent caliper placement, increasing measurement error. Above 84 mm, the growing fetus begins to curl, making a true straight-line crown-to-rump measurement impossible; clinicians then use biparietal diameter and head circumference for ongoing gestational dating.
Can different sonographers get different CRL measurements for the same fetus?
Yes, inter-observer variability is a well-documented limitation of CRL measurement. Studies show that two sonographers measuring the same fetus can obtain CRL values differing by 2 to 5 mm, equivalent to approximately 3 to 6 days of gestational age. Standardized technique reduces this variability: operators should obtain a true mid-sagittal plane, avoid fetal hyperflexion, and average at least three separate calipered measurements. High-resolution equipment and experienced operators further minimize discrepancies between readings.
How does CRL-based gestational age dating compare to last menstrual period (LMP) dating?
CRL-based ultrasound dating is more accurate than LMP dating because LMP relies on patient recall and assumes a regular 28-day cycle with ovulation on day 14. Cycle length and ovulation timing vary widely between individuals, introducing errors of one to two weeks. When CRL and LMP estimates disagree by more than 5 to 7 days in the first trimester, clinical guidelines recommend adopting the ultrasound-based gestational age as the official estimate, particularly in patients with irregular cycles or uncertain LMP dates.