Early pregnancy dating — Ultrasound

USRobinson–Fleming CRL / ISUOG

First-trimester ultrasound for gestational-age dating and viability.

Applies to

First-trimester ultrasound for gestational-age dating

Does not apply to

Second- and third-trimester dating (use biometry: BPD, HC, AC, FL)
Diagnosing pregnancy failure from a single measurement (apply strict criteria and follow-up)

What to report

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1.Location
Confirm an intrauterine gestational sac; exclude features of ectopic pregnancy.
Pitfall: A pseudogestational sac (central, no double decidual sign) can mimic an intrauterine pregnancy.
2.Crown–rump length
When an embryo is present, date by CRL — the most accurate first-trimester measurement. Prefer CRL over LMP when they disagree beyond the accepted margin.
3.Mean sac diameter
Before an embryo is measurable, report the mean of three orthogonal sac diameters.
4.Cardiac activity
Presence and rate of cardiac activity; an embryo with CRL ≥ 7 mm and no heartbeat meets a criterion for failure.
5.Yolk sac and adnexa
Yolk sac presence, number of gestational sacs (chorionicity), and adnexa/free fluid.

Scores & calculators

Gestational age from CRL

Gestational age from crown–rump length (Robinson–Fleming).

Crown–rump length

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Robinson–Fleming: GA (days) = 8.052 × √(1.037 × CRL) + 23.73, with CRL in mm. Most accurate in the first trimester (CRL ≈ 7–84 mm); CRL dating is preferred over LMP when they differ by more than the accepted margin.

Gestational age from MSD

Gestational age from mean sac diameter (before an embryo is visible).

Mean sac diameter

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GA (days) ≈ mean sac diameter (mm) + 30. Use only before a measurable embryo appears; once a CRL is available, date by CRL. MSD alone should not be used to diagnose pregnancy failure.

Sources

CRL dating — Robinson HP, Fleming JEE. Br J Obstet Gynaecol (1975)

Content review: Content pending clinical review · 2026-07-10

Educational reference for trained clinicians. Verify every result against the cited original guideline before use. Not a substitute for clinical judgment or medical advice.