What is the rate of pseudosac (false gestational sac) in ectopic pregnancy?

Rate of Pseudosac in Ectopic Pregnancy

Pseudogestational sacs occur in approximately 5-10% of ectopic pregnancies, with more recent evidence suggesting the lower end of this range at around 5%. 1, 2

Understanding Pseudogestational Sacs

A pseudogestational sac (also called a pseudosac) is an intrauterine fluid collection that can mimic a true gestational sac in patients with ectopic pregnancy. This finding can create diagnostic challenges when evaluating early pregnancy.

• Pseudosacs represent a collection of fluid within the endometrial cavity, not an actual embryonic structure 1
• They lack the double decidual sign and yolk sac that characterize true intrauterine gestational sacs 3
• The presence of a pseudosac can potentially lead to misdiagnosis of an intrauterine pregnancy when an ectopic pregnancy exists 4

Incidence and Clinical Significance

Recent evidence provides more precise data on pseudosac occurrence:

• In a 2022 retrospective cohort study of 1,236 women with pregnancy of unknown location, only 8 of 162 ectopic pregnancies (4.9%) had an intrauterine fluid collection that could be considered a pseudosac 5
• Earlier studies from the 1990s reported pseudosac rates of approximately 19% in ectopic pregnancies 2
• The presence of an intrauterine fluid collection significantly decreases the likelihood of ectopic pregnancy (adjusted relative risk 0.09; 95% CI 0.05-0.19) 5

Diagnostic Considerations

When evaluating for possible ectopic pregnancy, several ultrasound findings are important to distinguish pseudosacs from true gestational sacs:

• The identification of a yolk sac within an intrauterine fluid collection virtually excludes the possibility of an ectopic pregnancy 3
• A double decidual sign is present in 92% of normal gestational sacs but only 33% of pseudogestational sacs 3
• The size of the intrauterine fluid collection cannot reliably distinguish between a true gestational sac and a pseudosac 5

Clinical Implications

Understanding the rate and significance of pseudosacs has important implications for clinical practice:

• The presence of an intrauterine fluid collection without a yolk sac or embryo should not be used alone to rule out ectopic pregnancy 6
• Clinical guidelines recommend not using β-hCG values alone to exclude the diagnosis of ectopic pregnancy in patients with indeterminate ultrasound findings 6
• Specialty consultation or close outpatient follow-up is recommended for all patients with indeterminate pelvic ultrasound findings 6

Common Pitfalls

Several potential pitfalls exist in the diagnosis of ectopic pregnancy when pseudosacs are present:

• Misidentifying a pseudosac as a true intrauterine gestational sac can lead to delayed diagnosis of ectopic pregnancy 1, 4
• Some pseudosacs may even contain structures that mimic embryos ("pseudoembryos"), further complicating diagnosis 4
• Relying solely on the presence of an intrauterine fluid collection without considering other clinical and sonographic findings can lead to diagnostic errors 5

In summary, while pseudosacs occur in approximately 5-10% of ectopic pregnancies, their clinical significance has diminished with improved ultrasound technology and understanding. The presence of an intrauterine fluid collection should be interpreted within the entire clinical context, including other sonographic findings, to avoid incorrect or delayed diagnosis of ectopic pregnancy.