First and second year — the floor first, then every step
Transvaginal measurements decide: crown–rump length without a heartbeat, or mean sac diameter without an embryo, at the diagnostic thresholds. Below them the finding is "suspicious" and needs a repeat scan — never a diagnosis.
Orient first
A wrong diagnosis of failure ends a wanted pregnancy, so the diagnostic thresholds are set for specificity near 100%.
Commonly used SRU criteria: CRL ≥ 7 mm with no heartbeat, or mean sac diameter ≥ 25 mm with no embryo, is diagnostic (verify against the local guideline — some use slightly different cut-offs).
Findings below those thresholds are "suspicious for" failure and need a follow-up scan at an interval.
Acquire the study
Transvaginal ultrasound; M-mode for cardiac activity (no colour or spectral Doppler on the embryo in the first trimester).
The manoeuvre
Measure the crown–rump length in the true mid-sagittal plane (average of three).
Look for cardiac activity with M-mode.
Mean sac diameter: average of three orthogonal sac dimensions.
Yolk sac: present and its size; amnion.
Compare with any prior scan and the interval since — absent embryo with heartbeat after an expected interval is also diagnostic.
What confirms it
A diagnostic criterion met: CRL at or above the threshold without cardiac activity, or MSD at or above the threshold without an embryo, or interval criteria.
What licenses you to exclude it
A pregnancy of uncertain viability is not a failed pregnancy — recommend the follow-up scan.
The classic misread
Diagnosing failure below the thresholds.
Using colour Doppler on the embryo.
Missing an ectopic pregnancy by calling an intrauterine pseudosac an early sac.
Reference values
Each value carries the caveat that keeps it from being misused. Normal limits and diagnostic criteria are kept apart on purpose: a disease cut-off read as a normal range is the more dangerous mistake.
Normal limits
Gestational sac · Expected early sac growth rate
approximately 1 mm per day in mean sac diameter in the early first trimester
Growth-rate rules are supportive, not diagnostic — inter-observer error on a 2–3 mm structure swamps a day of growth, which is why the SRU intervals are set a week or more apart.
USG
Yolk sac · Yolk sac diameterup to about 6 mm before 10 weeks
Transvaginal ultrasound; inner-to-inner diameter at its widest.
An enlarged, calcified or irregular yolk sac is associated with, but does not by itself establish, pregnancy failure — it is a soft marker, and the consensus criteria above are what make a definitive diagnosis.
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Diagnostic criteria
Gestational sac · Mean sac diameter without a yolk sac or embryo
a sac of 25 mm or more with no embryo is diagnostic of pregnancy failure
Transvaginal ultrasound. Mean of three orthogonal internal sac diameters, measured inner-to-inner.
⚠️ THE THRESHOLDS WERE DELIBERATELY SET TO BE 100% SPECIFIC, because the cost of wrongly diagnosing failure is the loss of a wanted pregnancy. Anything short of the criteria is "suspicious for" and warrants an interval scan, never a definitive call.
USG
Gestational sac · β-hCG level at which an intrauterine sac should be visible
on transvaginal ultrasound an intrauterine gestational sac is reliably expected only above roughly 3500 IU/L in a singleton
⚠️ The older 1000–2000 IU/L zone caused methotrexate to be given to viable pregnancies — that is WHY the figure moved. A single hCG below the threshold with an empty uterus decides nothing; twins and dating error break the rule in both directions. Versioned criterion — verify against the current edition before clinical use.
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Embryo · Crown–rump length without cardiac activity
a CRL of 7 mm or more with no cardiac activity is diagnostic of pregnancy failure
Transvaginal ultrasound. Longest straight-line embryonic length, excluding the yolk sac and limbs.
⚠️ SPECIFICITY WAS CHOSEN OVER SENSITIVITY BY DESIGN. Below 7 mm, absent cardiac activity is suspicious and requires a follow-up scan. M-mode confirmation is preferred; spectral Doppler over an early embryo is avoided on ALARA grounds.
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Embryo · Heartbeat expectation by CRL (SRU)
a CRL of 7 mm or more with no cardiac activity is diagnostic of pregnancy failure — the same figure as the registered CRL-without-heartbeat entry. Below 7 mm, absent cardiac activity is suspicious and needs an interval scan, never a definitive failure call. A sac of 25 mm or more with no embryo is the companion MSD failure criterion
Do not invent a new millimetre. The diagnostic and suspicious CRL/MSD thresholds are listed as separate reference values — this entry exists so a reader does not collapse "no heartbeat yet" into "failed pregnancy" below those thresholds. Versioned criterion — verify against the current edition before clinical use.
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See it on real cases
Direct links to Radiopaedia — the reference article and worked cases with their images. Each opens on Radiopaedia.