What is an Anembryonic Pregnancy?
Anembryonic pregnancy is a type of early pregnancy loss where a gestational sac develops but no embryo forms, diagnosed when the gestational sac measures ≥25 mm without an embryo on transvaginal ultrasound, or when serial examinations show persistent absence of an embryo at appropriate time intervals. 1, 2
Modern Terminology
- The term "anembryonic pregnancy" has replaced the outdated and potentially hurtful term "blighted ovum" according to the 2025 Society of Radiologists in Ultrasound consensus guidelines. 1
- This terminology change reflects a broader shift away from language like "pregnancy failure" toward more compassionate terms such as "early pregnancy loss (EPL)." 1
- The American College of Obstetricians and Gynecologists endorses this updated terminology to improve patient communication and reduce emotional distress. 2
Diagnostic Criteria
Transvaginal ultrasound establishes the diagnosis using specific measurements: 1, 2
- Mean sac diameter (MSD) ≥25 mm with no visible embryo 1, 2
- Absence of an embryo ≥14 days after initial visualization of the gestational sac 1
- MSD 16-24 mm without an embryo or absence of embryo 7-13 days after visualized gestational sac are considered "concerning for EPL" but not yet diagnostic 1, 3
Underlying Pathophysiology
- Chromosomal abnormalities account for 50-60% of early pregnancy losses, including anembryonic pregnancies. 4
- Trisomies 16 and 22 are particularly prevalent in anembryonic pregnancies, suggesting genes on these chromosomes are responsible for early arrest of embryonic development. 5
- Environmental factors such as elevated plasma lead levels may contribute to risk, though additional cofactors are likely involved. 6
Classification Within Early Pregnancy Loss
- Anembryonic pregnancy falls under the category "Diagnostic of EPL" in the current classification system. 1
- This distinguishes it from other EPL subtypes including embryonic/fetal demise (where an embryo forms but cardiac activity ceases), incomplete EPL, EPL in progress, and completed EPL. 1
Critical Diagnostic Pitfalls to Avoid
- Never diagnose anembryonic pregnancy based on transabdominal ultrasound alone—transvaginal ultrasound is mandatory for accurate measurement. 2, 3
- Ensure accurate pregnancy dating before making the diagnosis, as incorrect dates can lead to premature diagnosis when the pregnancy is actually viable but earlier than expected. 2
- Detection of an anembryonic pregnancy should not preclude evaluation for simultaneous ectopic pregnancy, as heterotopic pregnancy can occur even without assisted reproductive technology. 7
- Serial β-hCG measurements showing failure to rise appropriately or declining levels support the diagnosis when ultrasound findings are equivocal. 4, 3