What ultrasound findings are characteristic of a blighted ovum?

Ultrasound Findings in Blighted Ovum (Anembryonic Pregnancy)

The primary ultrasound finding in a blighted ovum is a gestational sac measuring 25 mm or larger without an embryo, or absence of an embryo on serial examinations obtained at discriminatory time intervals. 1

Key Ultrasound Characteristics

Definitive Diagnostic Criteria

• Mean Sac Diameter (MSD) ≥25 mm with no embryo 1
• No embryo visible on follow-up ultrasound after appropriate interval 1
• Gestational sac with normal location in uterine cavity 1

Additional Findings That May Be Present

• Well-defined gestational sac with a hyperechoic rim
• Absence of yolk sac despite adequate gestational sac size
• Normal trophoblastic reaction around the sac (though this alone cannot rule out anembryonic pregnancy) 2
• Possible irregular contour of the gestational sac in some cases

Diagnostic Classification

According to the Society of Radiologists in Ultrasound (SRU) consensus, the term "blighted ovum" should be avoided in favor of more precise terminology 1:

1. Diagnostic of Early Pregnancy Loss (EPL) when:

   • MSD ≥25 mm with no embryo
   • Absence of embryo on serial examinations at appropriate intervals

2. Concerning for EPL when:

   • MSD 16-24 mm with no embryo
   • Absence of embryo 7-13 days after visualization of gestational sac without yolk sac

Clinical Implications

Anembryonic pregnancy represents a form of early pregnancy loss where the embryo either never developed or stopped developing very early, despite continued growth of the gestational sac. This condition accounts for approximately 50-75% of first-trimester miscarriages 3.

Important Caveats

• Avoid premature diagnosis: A single ultrasound showing an empty gestational sac <25 mm is not sufficient to diagnose anembryonic pregnancy 2
• Follow-up is essential: For gestational sacs measuring 16-24 mm without an embryo, follow-up ultrasound in 7-10 days is recommended before making a definitive diagnosis 1
• Transvaginal ultrasound is preferred: Offers better resolution for accurate measurement of gestational sac and detection of early embryonic structures
• Correlation with hCG levels: Should be considered alongside ultrasound findings, though the SRU guidelines emphasize specific ultrasound criteria for diagnosis

Differential Considerations

• Very early normal pregnancy (if gestational sac <25 mm)
• Pseudogestational sac of ectopic pregnancy (though this typically appears as fluid in the endometrial cavity rather than a true gestational sac) 1
• Pregnancy of unknown location requiring follow-up

Management Implications

The definitive ultrasound diagnosis of anembryonic pregnancy (MSD ≥25 mm with no embryo) allows clinicians to counsel patients appropriately and discuss management options including expectant management, medical management with misoprostol, or surgical evacuation 4.

Remember that the Society of Radiologists in Ultrasound consensus recommends using the term "anembryonic pregnancy" or "diagnostic of EPL" rather than "blighted ovum" in modern practice, as this terminology is more precise and less potentially distressing to patients 1.