Calculators · Obstetrics & Gynaecology · Formula

Gestational age and due date from crown–rump length (CRL)

Also called CRL dating, first trimester dating scan, INTERGROWTH-21st CRL, Robinson CRL formula

Gestational age from a first-trimester CRL (15–95 mm) by the INTERGROWTH-21st equation, with the Robinson–Fleming 1975 value alongside, and the due date if you enter the scan date.

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Formula and scoring

INTERGROWTH-21st: GA (days) = 40.9041 + 3.21585 × CRL^0.5 + 0.348956 × CRL; SD of GA = 2.39102 + 0.0193474 × CRL (CRL in mm, 15–95 mm). Robinson–Fleming: GA (days) = 8.052 × √CRL + 23.73. EDD = scan date + (280 − GA days).

Exam and ward pearls

  • CRL is the most accurate dating method: Robinson & Fleming estimated ±4.7 days (95%) from one measurement and ±2.7 days from three.
  • The INTERGROWTH-21st equation was built from CRLs measured at 9+0 to 13+6 weeks and is defined for CRL 15–95 mm.

When not to rely on it

  • Some papers quote Robinson's equation with CRL multiplied by 1.037 (a correction for the 1975 machine calibration error); this calculator uses the uncorrected form quoted by most recent papers. The two differ by about 1 day.
  • Whether the scan date should replace the LMP date depends on the size of the discrepancy and the gestation; follow your unit's redating policy.

Source

  1. Papageorghiou AT, Kennedy SH, Salomon LJ, et al. International standards for early fetal size and pregnancy dating based on ultrasound measurement of crown-rump length in the first trimester of pregnancy. Ultrasound Obstet Gynecol. 2014;44(6):641-8. doi:10.1002/uog.13448 · PMID 25044000
  2. Robinson HP, Fleming JE. A critical evaluation of sonar "crown-rump length" measurements. Br J Obstet Gynaecol. 1975;82(9):702-10. doi:10.1111/j.1471-0528.1975.tb00710.x · PMID 1182090
  3. Zhang Y, Niu Z, Meng H, et al. Gestational age reference from crown-rump length during 11-14 weeks: a population-based multicenter cohort study in China. BMC Pregnancy Childbirth. 2025;25:214 (table quoting Robinson 8.052 × CRL^0.5 + 23.73). doi:10.1186/s12884-025-07295-8 · PMID 40011832

Formula and cut-offs checked against these sources on 8 Nov 2026. Educational tool for clinicians; it does not replace clinical judgement.

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