At what week of gestation are human chorionic gonadotropin (hCG) levels typically assessed to diagnose a blighted ovum?

Diagnosing Blighted Ovum: Timing of hCG Assessment

Blighted ovum is best diagnosed using ultrasound criteria rather than relying solely on hCG levels, with definitive diagnosis typically made between 6-9 weeks of gestation when ultrasound can clearly demonstrate a mean sac diameter ≥25 mm with no embryo, or absence of embryo with cardiac activity ≥14 days after visualization of a gestational sac with no yolk sac. 1

Diagnostic Approach for Blighted Ovum

Ultrasound Criteria (Primary Diagnostic Method)

• Definitive criteria for diagnosis of early pregnancy loss/blighted ovum 1:

  • Mean sac diameter (MSD) ≥25 mm with no embryo
  • Absence of embryo with cardiac activity ≥14 days after visualization of gestational sac with no yolk sac
  • Embryonic crown-rump length (CRL) ≥7 mm with no cardiac activity

• Concerning criteria requiring follow-up 1:

  • MSD 16-24 mm with no embryo
  • Absence of embryo with cardiac activity 7-13 days after visualizing gestational sac with no yolk sac
  • CRL <7 mm with no cardiac activity

Timing of Ultrasound and hCG Assessment

• A gestational sac should be visible at approximately 4.5-5 weeks of gestation (MSD of 2-3 mm) 1
• Yolk sac should be visible when the gestational sac is >8 mm 1
• Cardiac activity should be present and visible by 6 weeks gestational age 1
• The discriminatory level of hCG (level at which a gestational sac should be seen) is approximately 3000 mIU/mL 1

Role of hCG in Diagnosis

While ultrasound is the primary diagnostic tool, hCG can provide supporting information:

• Before 8-9 weeks, hCG levels may still be in normal range despite a blighted ovum 2, 3
• After 9 weeks, abnormal hCG levels are more consistently associated with pregnancy failure 4
• By 10 weeks, nearly all cases of blighted ovum show abnormal gestational sac diameters, though hCG may occasionally remain normal until 14 weeks 2

Important Clinical Considerations

Avoiding Premature Diagnosis

• When findings are equivocal, a follow-up ultrasound in 7-14 days is recommended 1
• The Society of Radiologists in Ultrasound advises against premature diagnosis without meeting established criteria 1

Diagnostic Accuracy

• Ultrasound alone gave a correct prognosis in 78% of threatened abortion cases after a single examination 5
• Combined application of ultrasound and hCG levels improves diagnostic accuracy 4
• Transvaginal ultrasound is superior for visualizing early pregnancy structures 1

Potential Pitfalls

• In multiple gestations, one sac may be blighted while others develop normally 1
• Heterotopic pregnancies (simultaneous intrauterine and ectopic pregnancies) are rare but possible 1
• Intracavitary fluid may mimic a gestational sac but can be distinguished by shape, contents, or location 1

Follow-up After Diagnosis

• Follow-up ultrasound is needed to confirm complete expulsion for expectant or medical management
• hCG levels should be monitored until they return to normal 1
• Management options include expectant management, medical management, or surgical management (suction D&C) based on patient preferences and clinical circumstances 1

In summary, while hCG levels can provide supporting information, the diagnosis of blighted ovum is primarily based on ultrasound criteria, with definitive diagnosis typically possible between 6-9 weeks of gestation.