First and second year — the floor first, then every step
The question is not "is there an adnexal mass" — it is "is there a definite INTRAUTERINE pregnancy", and a pseudosac is not one.
Orient first
The logic runs the other way round from most searches: a definite intrauterine pregnancy makes ectopic very unlikely, so you look for the IUP first and hardest.
A definite IUP means a gestational sac containing a YOLK SAC or an embryo. A round fluid collection alone is not enough.
The PSEUDOSAC is the trap: a central, ovoid collection of fluid in the endometrial cavity with a single thin rim, caused by decidual reaction in an ectopic. A true sac is ECCENTRIC, embedded in the decidua, with a double decidual ring.
Interpretation always pairs with the βhCG and the discriminatory zone — the scan alone does not carry the decision.
Acquire the study
Transvaginal ultrasound is the study; transabdominal is a supplement for a high uterus or a large mass.
Empty bladder for transvaginal. Survey the whole pelvis including both adnexa, the pouch of Douglas, and both paracolic gutters and Morison pouch for free fluid.
Use colour Doppler for the ring of fire, at low-flow settings.
The manoeuvre
Look in the uterus first: is there a sac, is it ECCENTRIC, does it contain a yolk sac or embryo, is there a double decidual ring?
Measure the endometrial thickness and describe the cavity contents.
Examine both adnexa systematically, separately from the ovary — an ectopic is usually SEPARATE from the ovary and moves independently on gentle probe pressure (the "sliding sign"), whereas a corpus luteum moves with it.
Look for a tubal ring: an echogenic ring around a small sac, often with a "ring of fire" of peripheral vascularity.
Look for free fluid, and characterise it — anechoic versus echogenic. ECHOGENIC free fluid means haemoperitoneum.
Check for haemoperitoneum beyond the pelvis: Morison pouch and the paracolic gutters. Fluid up to the liver indicates a substantial bleed.
Consider the sites that are easy to miss: interstitial/cornual (thin overlying myometrium, interstitial line sign), caesarean scar, cervical, and ovarian.
What confirms it
An extrauterine gestational sac containing a yolk sac or embryo is diagnostic.
An adnexal mass separate from the ovary with a tubal ring, in a positive pregnancy test with an empty uterus, is an ectopic until proven otherwise.
What licenses you to exclude it
A definite intrauterine pregnancy (sac WITH yolk sac or embryo) makes ectopic very unlikely — but not impossible, and heterotopic pregnancy must be considered after assisted conception.
⚠️ An empty uterus with no adnexal finding is a PREGNANCY OF UNKNOWN LOCATION, not a negative scan. It requires βhCG follow-up and must be reported as such.
The classic misread
Calling a pseudosac an intrauterine pregnancy and discharging an ectopic.
Mistaking a corpus luteum for an ectopic — the corpus luteum is INTRAovarian and moves with the ovary.
Not looking above the pelvis for haemoperitoneum.
Missing an interstitial ectopic because it looks intrauterine — check the myometrial mantle around it.
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
Pelvic free fluid · Physiological free fluid
a small volume of anechoic cul-de-sac fluid is physiological in reproductive-age women, particularly mid-cycle and in the luteal phase
Echogenic fluid, a volume clearly beyond a trace, fluid in a postmenopausal pelvis, or fluid with a positive pregnancy test each changes the meaning entirely — haemoperitoneum and ectopic rupture sit behind this finding.
USG · CT
Diagnostic criteria
Early pregnancy · Findings SUSPICIOUS for (not diagnostic of) pregnancy failure
a named criteria set: mean sac diameter 16–24 mm without an embryo, no heartbeat in an embryo under 7 mm CRL, and absent embryo 7–13 days after a sac-only scan are suspicious and mandate follow-up, not a verdict
The whole architecture of these criteria is the separation of SUSPICIOUS from DIAGNOSTIC — collapsing the two is the error the consensus exists to prevent. When suspicious, the correct report is a dated follow-up scan, not a diagnosis. Versioned criterion — verify against the current edition before clinical use.
USG
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.
USG
See it on real cases
Direct links to Radiopaedia — the reference article and worked cases with their images. Each opens on Radiopaedia.