A 28-year-old woman with irregular menstrual cycles presents for dating ultrasound at what she believes is 12 weeks of gestation. Transvaginal ultrasound shows a single intrauterine gestational sac measuring 18 mm in mean sac diameter (MSD) with no yolk sac or fetal pole visible. The endometrial thickness is 12 mm. What is the most accurate interpretation and next management step?

## Dating and Viability Assessment in First Trimester **Key Point:** According to current ACOG/RCOG/SRU guidelines, an MSD ≥16 mm with no embryo (fetal pole) visible on transvaginal ultrasound is diagnostic of a **failed pregnancy (anembryonic pregnancy / blighted ovum)**. Similarly, an MSD ≥8 mm with no yolk sac is also diagnostic of failure. These thresholds were updated (from the older >25 mm cutoff) to reduce false-positive diagnoses of miscarriage while still allowing definitive diagnosis when criteria are clearly met. ### Updated Diagnostic Criteria for Pregnancy Failure (ACOG/SRU 2012 Consensus) | Finding | Diagnostic Threshold (TVS) | Interpretation | |---|---|---| | No yolk sac | MSD ≥ 8 mm | Failed pregnancy | | No embryo (fetal pole) | MSD ≥ 16 mm | Failed pregnancy (anembryonic) | | No cardiac activity | CRL ≥ 7 mm | Embryonic demise | | No embryo at follow-up | MSD growth <1 mm/day over 7–10 days | Failed pregnancy | **High-Yield:** In this vignette, MSD = 18 mm with **no yolk sac and no fetal pole** on transvaginal ultrasound. This **exceeds both thresholds** (≥8 mm without yolk sac AND ≥16 mm without fetal pole), meeting definitive criteria for anembryonic pregnancy. A diagnosis of failed pregnancy can be made on a **single scan** when these thresholds are clearly exceeded. ### Why the Other Options Are Incorrect - **Option A (Ectopic):** An intrauterine gestational sac is already documented; ectopic pregnancy is effectively excluded. Pelvic MRI is not indicated. - **Option B (7–8 weeks normal):** An MSD of 18 mm corresponds to ~9–10 weeks, not 7–8 weeks. At 7–8 weeks, MSD would be ~8–12 mm. Labeling this as "early normal" is factually incorrect. - **Option C (Repeat in 1 week):** While repeat ultrasound is appropriate when findings are equivocal, the MSD of 18 mm here **clearly exceeds** the diagnostic threshold for anembryonic pregnancy. Delaying diagnosis is not warranted when criteria are definitively met. The older threshold of >25 mm has been superseded. **Clinical Pearl:** The 2012 SRU/ACR/ACOG consensus statement lowered the diagnostic threshold for anembryonic pregnancy to MSD ≥16 mm (no embryo) and MSD ≥8 mm (no yolk sac) on transvaginal ultrasound, replacing the older >25 mm cutoff. These conservative thresholds were chosen to avoid false-positive diagnoses while still permitting definitive diagnosis when clearly exceeded. **Management:** Once anembryonic pregnancy is diagnosed, options include expectant management, medical management (misoprostol), or surgical evacuation (uterine aspiration). Immediate evacuation is one valid option, though shared decision-making with the patient is standard practice. *Reference: Doubilet PM et al. "Diagnostic Criteria for Nonviable Pregnancy Early in the First Trimester." NEJM 2013; 369:1443–1451. Also: ACOG Practice Bulletin No. 200.*