4 Pitfalls of Bedside Ultrasonography During First Trimester Pregnancy

One of the more common indications for the use of bedside ultrasound (US) is to evaluate patients who present to the emergency department (ED) with pain or bleeding during the first trimester of pregnancy. When performing this study, providers should be aware of several potential pitfalls that pose significant risk to both the patient and the provider.

Background: Ultrasonography during first trimester pregnancy

Emergency medicine (EM) literature and EM professional organizations support the use of bedside US in the evaluation of a first-trimester pregnant patient. McRae et al. found that when providers were able to identify an intrauterine gestational sac with a yolk sac or fetal pole, the specificity of bedside US for identifying an intrauterine pregnancy (IUP) was 92-100%. There was only one case of false positive finding where an ectopic pregnancy was initially categorized as an IUP. Training for providers in the ED involved 10-50 proctored scans in addition to ~10 hours of formal didactics.¹ Current ACEP US guidelines call for a total of 80 hours of training in bedside US with a minimum of 2 weeks dedicated. In addition residents must perform a minimum of 150 US scans in “critical” or “life-saving” scenarios.²

Not everyone supports emergency physicians performing this study

There is little agreement between EM, Radiology, and OB/GYN in terms of the training that is necessary to perform first trimester bedside US.³ Compared to ACEP recommendations, training guidelines put forth by the American College of Radiology (ACR), the American College of Obstetrics and Gynecology (ACOG), the American Institute of Ultrasound in Medicine (AIUM), and the Society of Radiologists in Ultrasound (SRU) call for a greater level of exposure in order to obtain competency.⁴

The AIUM requires evidence of performing, evaluating, and interpreting at least 300-500 sonograms over a 3 year training period comprising a minimum of 3 months of US experience. Once “competency” is achieved, recommendations from the ACR-SPR-SRU joint guidelines and the AIUM states that providers must perform a minimum of 100 diagnostic obstetric USs per year to maintain their skill sets.⁵

Guidelines from other organizations do not specifically address the specific diagnostic questions that are asked with obstetric US between various specialties. For OB/GYN and radiologists, an obstetric US may be used to evaluate a significant number of parameters and to rule out a wide variety of potential complications. When used in the ED, obstetric US should be used to answer the single binary question: “Is there an intrauterine pregnancy?” It is unclear if, when the scope of the exam is limited to this question, providers could attain competency with less intensive training.

Given a more limited scope of the first trimester pregnancy US exam, EM providers should remember 4 major pitfalls:

1. Beware an “indeterminate” ultrasound

Providers should be cautious of any indeterminate or unclear US findings as these findings are often associated with a high rate of ectopic pregnancy. The definition of an indeterminate pregnancy is heterogeneous in the literature. For the purposes of EM providers, a definition of indeterminate pregnancy may be considered for any US that fails to demonstrate a yolk sac, fetal pole, or fetal heartbeat. Mateer et al. examined the role of emergency provider bedside US in patients who were diagnosed with an ectopic pregnancy after initially being discharged home. All of the discharged patients were noted to have either “abnormal IUP” or “nondiagnostic” findings on their initial US.⁶

Tayal et al. looked at outcomes of 1,490 patients who underwent a bedside US performed by an EP:⁷

• 70% were diagnosed with an IUP
• 8% were diagnosed with fetal demise
• 2% were diagnosed with ectopic pregnancy
• Remaining 20% were categorized as “indeterminate”

All non-IUPs underwent a formal US and OB/GYN consultation. Patients who initially had an “indeterminate” US overall had poor outcomes with a 53% rate of fetal demise and 15% rate of ectopic pregnancy. Furthermore 36% of the ectopics required surgical management.

Given this high rate of complications in patients with an initially “indeterminate” US, providers should obtain a formal US study or arrange 48-hour follow up with a gynaecologist in any patient who has findings on bedside US other than a classic IUP.⁷

2. Beware IUP mimics

In a recent meta-analysis, Stein et al. reported a pooled sensitivity of 99.3%, a negative predictive value of 99.96%, and a negative likelihood ratio of 0.08 for the use of bedside US performed by the ED provider in ruling out an ectopic pregnancy. The authors concluded that the presence of an IUP on bedside US reliably rules out an ectopic pregnancy. Unfortunately there are several scenarios where an abnormal pregnancy may closely resemble an IUP on bedside US which could lead to misdiagnosis. This may include, but not limited to, fetal demise, anembyronic pregnancies, cervical ectopic, or interstitial pregnancies.

Special consideration should be given to cervical ectopic and interstitial pregnancies. Cervical ectopics and interstitial pregnancies account for ~4% of all ectopics yet are associated with an increased risk of significant bleeding, emergency hysterectomy and maternal death. To diagnose these complications providers must be able to identify findings such as irregular placement of the gestational sac or incomplete myometrial mantle. Few ED based protocols address these often subtle findings and in a system where emergency physicians may be credentialed to perform bedside US after 10 proctored scans, these providers may overlook subtle signs that are indicative of an interstitial pregnancy.^8,9

3. Beware heterotopic pregnancies

Heterotopic pregnancies, or the presence of both an ectopic and intrauterine pregnancy, are thought to occur in 1 in 4000-30,000 pregnancies during a natural conception cycle. Previous studies have reported  a ~5% rate of ectopic pregnancy in patients who are undergoing in-vitro fertilization therapy.¹⁰ In patients using assisted reproduction techniques the incidence of heterotopic pregnancy is reported to be as high as 1 in 100.¹¹ Transvaginal US has a reported 56% sensitivity in diagnosing heterotopic pregnancies at 5-6 weeks when performed by certified US technicians.¹² To date, there is no data to suggest that ED providers can reliably diagnose heterotopic pregnancy using bedside US. While the exact risk is somewhat unclear, patients using assisted reproductive technology seem to have a significant risk of developing a heterotopic pregnancy. ED providers should obtain a formal US when evaluating these patients due to their risk of heterotopic pregnancy.

4. Beware the BHCG “Discriminatory Zone”

The beta-HCG (BHCG) “discriminatory zone” was originally described in 1981 by Kadar as a means to predict the likelihood of detecting an IUP on US in relation to a quantitative BHCG level.¹³  In practice, at a BHCG of >1000 IU/mL and 4,000-6000 IU/mL, an IUP should be detectable by trans-vaginal or trans-abdominal US, respectively. Clinicians should NOT use the fact that a BHCG is less than the discriminatory zone cutoff to suggest that an US is not required given the unlikelihood of detecting an IUP on US. It has been documented in the literature of ectopic pregnancies presenting well below traditional discriminatory thresholds, potentially as low as 30 IU/mL.^14–16 In addition, a quantitative BHCG level cannot be utilized to rule out the possibility of an ectopic pregnancy or clinically differentiate an ectopic pregnancy from an early IUP.^7,17 All patients who present to the ED with a BHCG greater than zero should have an US performed.

Conclusion

Providers need to consider several potential pitfalls when evaluating a pregnant patient using bedside US.

1. Clinicians should recognize the focused scope of EM bedside obstetric US such that any indeterminate or unusual findings should be formally evaluated, because these are at higher risk for poor outcomes, such as fetal demise and ectopic pregnancies.
2. Any patient who has a significant risk of a heterotopic pregnancy, including those undergoing any type of fertility treatment, should have a formal US performed.
3. Any patient presenting with a positive urine BHCG must receive an US even if below the discriminatory zone.

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Author information

Brian Bauerband, MD

PGY-2 EM resident
Department of Emergency Medicine
University of Alabama at Birmingham

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