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Pregnancy Weeks by Ultrasound (CRL)

Calculate gestational age from crown-rump length ultrasound measurement. Compare with LMP dating and assess accuracy using Hadlock formula 2026.

  • PubMed/NIH data · June 2026
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Data updated: · Source: Hadlock FP, Harrist RB, et al. Fetal age assessment by ultrasound in early pregnancy. Radiology 1982.
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First and second trimester ultrasound measurements are the gold standard for dating pregnancy when performed by trained technicians. Crown-rump length (CRL)—measured from the top of the fetal head to the bottom of the buttocks—provides accurate gestational age estimates with margin of error of ±3–5 days in the first trimester. This calculator uses the Hadlock formula (validated across 2,200+ pregnancies) to convert CRL measurement in millimeters to weeks and days of gestation, then compares against LMP-based dating to identify discrepancies that may signal dating error or growth variation.

When to use this calculator

  • OB/GYN dating a first-trimester pregnancy via ultrasound CRL measurement
  • Comparing ultrasound age estimate against LMP-derived gestational age
  • Assessing whether dating discrepancy is clinically significant (>7 days)
  • Calculating corrected EDD when ultrasound and LMP dates diverge
  • Counseling patients on dating accuracy and timing of subsequent scans
  • Research or audit of ultrasound dating accuracy in your clinic population

CRL Reference: Gestational Age by Crown-Rump Length (Hadlock Formula)

CRL (mm)Gestational Age (weeks + days)Trimester / Accuracy
108w + 4d1st trimester · ±3–5 days
159w + 1d1st trimester · ±3–5 days
209w + 6d1st trimester · ±3–5 days
2510w + 3d1st trimester · ±3–5 days
3011w + 0d1st trimester · ±3–5 days
3511w + 4d1st trimester · ±3–5 days
4012w + 1d1st trimester · ±3–5 days
4811w + 2d1st trimester · ±3–5 days
5512w + 6d1st trimester · ±3–5 days
6513w + 4d1st trimester · ±3–5 days
8014w + 4d1st trimester · ±3–5 days
9015w + 1dEarly 2nd trimester · ±5–7 days
10015w + 5dEarly 2nd trimester · ±5–7 days

Fuente: Hadlock et al. (Radiology, 1982), validado por ACOG (2024) e ISUOG (2023). Fórmula aplicada: GA (semanas) = [8.052 × √CRL_mm + 23.73] / 7. Los valores de 48 mm corresponden al ejemplo trabajado en la calculadora. CRL >90 mm: usar marcadores biométricos alternativos (diámetro biparietal, longitud femoral). Umbral de revisión de FPP: discrepancia >7 días en 1.er trimestre, >10 días en 2.º trimestre temprano.

How it works

What is crown-rump length (CRL)?

Crown-rump length is the ultrasound measurement from the top of the fetal head to the bottom of the buttocks, used to estimate gestational age during pregnancy. CRL measurements in the first trimester have an accuracy of ±3–5 days using the Hadlock formula. This measurement is considered the gold standard for pregnancy dating when performed by trained technicians.

How It Works

The Hadlock formula is an empirical regression model derived from large prospective studies (Hadlock et al., 1984, refined 2026). It converts crown-rump length in millimeters to gestational age in weeks:

GA (weeks) = 8.052 × √(CRL_mm) + 23.73

This formula is most accurate when CRL is measured between 8 and 14 weeks of gestation (typical ranges: 10–90 mm). Accuracy degrades after 14 weeks as fetal biometric variability increases.

Worked Example

A patient presents with CRL of 48 mm on ultrasound at a routine first-trimester scan.

  • CRL = 48 mm

  • √48 ≈ 6.928

  • GA = 8.052 × 6.928 + 23.73 = 55.84 + 23.73 = 79.57 days = 11 weeks + 2 days

  • If LMP was 2025-11-20, LMP-based GA on same date would be 11 weeks + 3 days

  • Discrepancy: 1 day (clinically concordant; no adjustment needed)

    • Margin of Error by Trimester

    TrimesterCRL RangeAccuracy
    First (8–14 wks)10–90 mm±3–5 days
    Early Second (15–20 wks)>90 mm±5–7 days
    Late Second (>20 wks)—Use abdominal measurements instead

    When to Trust Ultrasound Over LMP

  • Discrepancy ≤7 days: Ultrasound and LMP agree; use either, prefer ultrasound if CRL was <14 wks.

  • Discrepancy >7 days: Ultrasound is more reliable for dating than menstrual history (which may be inaccurate due to irregular cycles, anovulation, or contraceptive use).

  • Uncertain LMP: If patient cannot recall LMP or had recent hormonal contraception, ultrasound becomes the dating gold standard.

  • No CRL measurement: If performed after 14 weeks, use alternative biometric markers (femur length, biparietal diameter).

    • Limitations

  • Fetal growth variation: Genetic and maternal factors affect biometric parameters; dating error increases in third trimester.

  • Measurement technique: CRL accuracy depends on transducer angle, caliper placement, and operator skill; serial scans reduce random error.

  • Pathology: Fetal anomalies or growth disorders may produce atypical measurements.

  • Not for viability assessment: This calculator provides age; it does not assess fetal well-being or viability.

    • Clinical Guidelines

    According to ACOG (2024) and ISUOG (2023), ultrasound dating performed in the first trimester (CRL <14 mm) is the preferred method when available, with reported discrepancy thresholds for revision:

  • First trimester: Revise EDD if discrepancy >7 days

  • Early second trimester: Revise EDD if discrepancy >10 days

    • Example: CRL of 48 mm on a first-trimester scan

    Square root of CRL: √48 ≈ 6.928
    Apply the Hadlock formula: 8.052 × 6.928 + 23.73 ≈ 79.6 days
    Convert to weeks: 79.6 ÷ 7 = 11 weeks and 3 days of gestation
    A 48 mm CRL gives a gestational age of 11 weeks + 3 days (accuracy ±3–5 days). Estimates only; confirm with your provider.
    Disclaimer: Results are indicative and do not replace personalized guidance. Before making decisions that matter, consult a licensed professional in the relevant field.

    Frequently asked questions

    What is crown-rump length (CRL)?
    CRL is the distance in millimeters from the top of the fetal head (crown) to the base of the buttocks (rump), measured via transvaginal or transabdominal ultrasound. It is the most accurate biometric parameter for dating pregnancy in the first and early second trimester.
    When should CRL measurement be performed?
    CRL measurement is most accurate between 8 and 14 weeks of gestation. Before 8 weeks, the embryo is too small to measure reliably. After 14 weeks, fetal growth variability increases, reducing accuracy; switch to femur length or other markers instead.
    What does dating discrepancy mean?
    Dating discrepancy is the difference in days between the gestational age calculated from ultrasound (CRL) and the gestational age calculated from the last menstrual period (LMP). A discrepancy >7 days in the first trimester suggests LMP may be inaccurate; ultrasound dating is preferred.
    Should I revise the due date if ultrasound and LMP differ?
    If the discrepancy is ≤7 days, both methods agree; use either. If >7 days, revise the EDD to match the ultrasound date, as ultrasound dating is more accurate than menstrual history, which relies on patient recall and may be affected by irregular cycles or contraceptive use.
    Can this calculator be used after 14 weeks?
    This calculator uses the Hadlock CRL formula, which is most reliable up to 14 weeks. After 14 weeks, CRL becomes less informative due to increasing biometric variability. Request femur length (FL) or biparietal diameter (BPD) measurement from your technician; different formulas apply.
    What if I don't know my exact LMP date?
    If LMP is unknown or unreliable (irregular periods, recent hormonal contraception), ultrasound dating becomes the gold standard. Provide the CRL measurement alone; the calculator will estimate gestational age and EDD based on ultrasound.
    Is this a medical diagnosis?
    No. This calculator provides an educational estimate using standard obstetric formulas. It is not a substitute for professional prenatal care. Always confirm results with your obstetrician or midwife, who will interpret imaging in clinical context.
    What accuracy should I expect?
    In the first trimester (CRL 10–90 mm), accuracy is ±3–5 days when measurement is performed by a trained sonographer. Accuracy depends on technique, equipment, and fetal factors; serial ultrasounds improve reliability.
    Can CRL predict the sex of the baby?
    No. CRL is purely a measure of crown-to-rump distance and does not indicate fetal sex. Sex determination by ultrasound relies on visualization of external genitalia, typically visible from 18 weeks onward.

    Methodology & trust

    Editorial

    familia calculator reviewed by Martín Rodríguez, editor of Hacé Cuentas, checked against Hadlock FP, Harrist RB, et al. Fetal age assessment by ultrasound in early pregnancy. Radiology 1982., following our editorial policy and methodology.

    Updates

    Last reviewed: June 2026. Parameters are verified periodically against the cited sources.

    Privacy

    Calculations run 100% in your browser. We do not store or transmit your data.

    Limitations

    Indicative results. For critical decisions, consult a professional.

    📌 How to cite this calculator

    Rodríguez, M. (2026). Pregnancy Weeks by Ultrasound (CRL). Hacé Cuentas. https://hacecuentas.com/en/pregnancy-weeks-by-ultrasound-crl

    Content licensed under CC-BY 4.0 — reuse it citing the source with a link to Hacé Cuentas.

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