What ultrasound size‑based criteria are used to diagnose an anembryonic (blighted‑ovum) pregnancy?

Anembryonic Pregnancy Diagnostic Criteria

A gestational sac measuring ≥25 mm in mean diameter without a visible embryo is diagnostic of anembryonic pregnancy (also called blighted ovum). 1, 2

Size-Based Diagnostic Thresholds

Definitive Criteria for Anembryonic Pregnancy

• Mean sac diameter (MSD) ≥25 mm without a visible embryo on transvaginal ultrasound is the gold standard for diagnosing anembryonic pregnancy, with 100% positive predictive value. 1, 2
• This threshold was increased from the previous 16 mm cutoff to maximize diagnostic certainty and avoid inadvertent harm to a viable embryo due to measurement variability. 1

Intermediate Findings Requiring Follow-Up

• MSD <25 mm without an embryo does NOT allow diagnosis of pregnancy loss—these cases require repeat ultrasound in 7–10 days. 1, 2
• MSD >8 mm typically shows a yolk sac if the pregnancy is viable. 1
• MSD ≥16 mm usually shows an embryo if the pregnancy is viable. 1
• An MSD ≥20 mm without a yolk sac or embryo predicted non-viability with 100% PPV in one large cohort, though the conservative 25 mm threshold remains the guideline standard. 3

Time-Based Diagnostic Criteria

When the initial scan shows MSD <25 mm, time-based follow-up criteria apply:

• If a yolk sac is present (but no embryo): Diagnose anembryonic pregnancy if there is no embryonic cardiac activity ≥11 days later. 1, 2
• If no yolk sac is present: Diagnose anembryonic pregnancy if there is no embryonic cardiac activity ≥14 days later. 1, 2
• Finding an empty gestational sac on two scans ≥7 days apart is highly predictive of miscarriage, regardless of growth rate. 4

Embryonic Demise vs. Anembryonic Pregnancy

To distinguish from embryonic demise (when an embryo is present but non-viable):

• Crown-rump length (CRL) ≥7 mm without cardiac activity confirms embryonic demise (not anembryonic pregnancy). 1, 2
• The CRL threshold was increased from 5 mm to 7 mm to account for measurement variability and maximize diagnostic certainty. 1
• Absence of cardiac activity in embryos <7 mm is worrisome but requires follow-up ultrasound in 7–10 days before confirming demise. 1

Critical Pitfalls to Avoid

• Never diagnose anembryonic pregnancy based solely on absence of a yolk sac or embryo when MSD is <25 mm—this can result in termination of a viable early pregnancy. 1, 2
• Do not use growth rates (MSD or CRL growth per day) as diagnostic criteria, as there is significant overlap between viable and non-viable pregnancies; no safe cutoff exists for MSD growth to exclude viability. 4
• Only 12% of early pregnancies fall into the intermediate zone (CRL 5–7 mm or MSD 16–25 mm) requiring additional follow-up under the new conservative criteria. 5

Terminology Recommendations

• Use "anembryonic pregnancy" or "early pregnancy loss" rather than outdated terms like "blighted ovum" or "pregnancy failure" when communicating with patients. 2, 6
• The term "cardiac activity" should be used instead of "heartbeat," "heart motion," or "viable" when describing first-trimester ultrasound findings. 2

Technical Considerations

• Transvaginal ultrasound is the reference standard for first-trimester evaluation, providing superior resolution compared to transabdominal scanning. 1, 7
• The discriminatory hCG level (approximately 1,000–2,000 mIU/mL for transvaginal ultrasound) indicates when a gestational sac should be visible, but hCG alone should never be used to diagnose anembryonic pregnancy. 8, 7
• Presence of a yolk sac within an intrauterine fluid collection is definitive evidence of intrauterine pregnancy and rules out pseudogestational sac. 1, 7