Early pregnancy ultrasound and hCG interpretation — structured SAQ (15 marks)

(a) Classification, management and interval (4 marks)

One mark each, maximum four. [1]

• Classification: intrauterine pregnancy of uncertain viability. There is an intrauterine sac, so this is not a pregnancy of unknown location, but the criteria for either viability or failure are not met.[1]
• The measurement is below the diagnostic threshold. A mean gestational sac diameter of 23 mm with no fetal pole is under the 25.0 mm transvaginal cut-off, so miscarriage cannot be diagnosed today.[1]
• Management: repeat transvaginal ultrasound a minimum of 7 days after the first, and warn that further scans may still be needed before a diagnosis can be made.[1]
• Reason for the interval: the cut-offs and the 7-day gap exist to make a false-positive diagnosis of miscarriage approach zero. Tell her explicitly that waiting for a repeat scan has no detrimental effect on the outcome of a viable pregnancy.[1][2]

(b) Four reasons reporting miscarriage today would be unsafe (4 marks)

One mark each, maximum four. [2][3]

• The measurement is below the criterion. Diagnosis requires a mean gestational sac diameter of 25.0 mm or more with no fetal pole on transvaginal scan.[1]
• The older cut-offs were demonstrably unsafe. With a mean sac diameter cut-off of 16 mm the false-positive rate for miscarriage was 4.4%, and at 20 mm it was still 0.5%; only at 21 mm or more were there no false positives in that cohort.[2]
• Measurement error crosses the threshold. Intra- and interobserver reproducibility of transvaginal sac measurement spans several millimetres, so a sac reported at 23 mm may genuinely be several millimetres larger or smaller.[3]
• Her dates are uncertain, and dates are not a viability criterion. Gestational age from the last menstrual period alone must not be used to determine whether a fetal pole or heartbeat should be visible.[1]

Additional credit for noting that the alternative to a 7-day wait is a second opinion on viability, and that either is acceptable once the 25.0 mm threshold is met. [1]

(c) Refuting the discriminatory zone (4 marks)

• State the correct label. An empty uterus with no adnexal findings and a positive pregnancy test is a pregnancy of unknown location, not an ectopic pregnancy. (1 mark)[7]
• State the rule. Serum hCG must not be used to determine the location of a pregnancy; it is used only to assess trophoblastic proliferation and guide management. (1 mark)[7]
• Give the evidence. In a prospective multicentre pregnancy of unknown location cohort, 39 of 358 women (11%) with an hCG above 3000 IU/L still had a viable intrauterine pregnancy. Separately, the hCG level at which a gestational sac would be predicted visible 99% of the time was measured at 3510 mIU/mL, far above the 1500 to 2000 most units quote. (1 mark)[5][6]
• Give the management. Paired serum hCG as near as possible to 48 hours apart and no earlier; act on the change band; written safety-net advice with a 24-hour contact; and a locating scan or clinical review as the band dictates. (1 mark)[7]

(d) True sac versus pseudosac (3 marks)

One mark each, maximum three. [7]

• Position: a true gestational sac is eccentrically placed within the endometrium; a pseudosac sits centrally in the cavity, in the midline.[7]
• Wall: a true sac shows the double decidual sign — a hypoechoic structure surrounded by two concentric echogenic rings; a pseudosac has a single thin outline.[7]
• Contents and evolution: a true sac goes on to contain a yolk sac and then an embryo; a pseudosac never does, is often teardrop-shaped, and may change shape with probe pressure.[7][1]