Gestational Age by Ultrasound (CRL)

Estimate gestational age from crown-rump length (CRL) using the Robinson-Fleming formula. Most accurate at 7–13 6/7 weeks.

For educational and clinical reference. Not a substitute for medical judgment. See the medical disclaimer.
Gestational Age
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References

  1. Robinson HP, Fleming JE. A critical evaluation of sonar "crown-rump length" measurements. Br J Obstet Gynaecol. 1975;82(9):702-710.
  2. ACOG Committee Opinion No. 700: Methods for Estimating the Due Date. Obstet Gynecol. 2017;129(5):e150-e154.

How to use

  1. Enter crown-rump length (CRL) in mm.
  2. Gestational age (weeks + days) updates instantly.
  3. Most accurate between 7 0/7 and 13 6/7 weeks.

Frequently asked questions

When is CRL most accurate for dating?

CRL is most accurate between 7 0/7 and 13 6/7 weeks of gestation, with a precision of ±5–7 days.

What formula does this use?

Robinson-Fleming: GA (days) = 8.052 × √(CRL_mm × 1.037) + 23.73.

Why is ultrasound dating preferred over LMP?

First-trimester ultrasound is more reliable than LMP when menstrual cycles are irregular, when LMP is uncertain, or in pregnancies conceived with assisted reproduction.

When should I switch from CRL to other measurements?

After 14 weeks, CRL becomes unreliable. Use biometry (BPD, HC, FL, AC) for dating in the second and third trimesters.

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