Pearls and Pitfalls in Early Pregnancy Evaluation
By Dr. Abdalmohsen Ababtain
A 21 year-old female presents to the Emergency Department (ED) with lower abdominal pain and vaginal spotting for one day. She is vitally stable and well appearing, with mild tenderness on abdominal exam. Since every woman of childbearing age is pregnant until proven otherwise, a serum pregnancy test is ordered from triage, which reveals a positive Beta-hCG of 3,200 IU/L. A transvaginal ultrasound (TVUS) shows no signs of intrauterine pregnancy (IUP), with normal adnexa. According to the most recent evidence, what is the most appropriate next step?
- Diagnose the patient with an ectopic pregnancy, and administer methotrexate in consultation with the obstetrics and gynecology (OBGYN) consultant
- Discharge the patient with instructions to return to the OBGYN clinic in 48 hours for a repeat serum beta-hCG
- Admit for emergency surgery
Does the Beta-hCG level matter?
Classically, the Beta-hCG serum level has been used to risk stratify patients presenting with a concern for ectopic pregnancy. “Discriminatory zones” have been defined, which are the levels of Beta-hCG at which you expect to see an IUP; 6,500 IU/L for transabdominal scans and 1,500-3,000 IU/L for transvaginal. Recent literature has questioned the usefulness of these levels .1,2
Despite the common belief that IUPs with a beta-hCG above the discriminatory zone should be observed on a pelvic ultrasound, a study by Wang et al showed otherwise. In an ED population of almost 300 women presenting for concerns of ectopic pregnancy, a beta-hCG level greater than 3,000IU/L had a sensitivity and specificity of around 35% and 58%, respectively, for identification of an IUP using a bedside pelvic ultrasound (transabdominal followed by a transvaginal, as needed). Phrased differently, the emergency physicians in this study were unable to visualize the IUP using bedside US in 26% of cases. 3 This is thought to be related to gestational age, operator skill and equipment, among other factors.
Several other explanations exist for a patient with a serum Beta-hCG above the discriminatory zone with no IUP. A complete miscarriage (abortion) with declining serum Beta-hCG is one, and the early development of multiple gestations is another.
In conclusion, solely using a beta-hCG level to differentiate between an IUP and an ectopic pregnancy is inadequate, and relying on traditionally defined discriminatory zones can result in missing the diagnosis of an ectopic pregnancy.
What is the role of ultrasound?
An ultrasound is a must for all symptomatic pregnant patients, according to the ACEP clinical policy released on September 2012.4 Patients with indeterminate abdominal scans should receive a transvaginal ultrasound.
An interesting distinction is that in emergency medicine literature, the definitive sign for pregnancy is cited as a gestational sac with yolk sac or fetal pole, in contrast to radiology literature, which sometimes considers the double decidual sign (which appears earlier than the gestational sac) as diagnostic of an IUP.4
Could frequent US harm the fetus?
Performing ultrasounds in pregnant patients has long been considered standard of care, and coupled with its widespread availability, healthcare providers incorrectly view it as a harmless process. It actually causes thermal and mechanical effects, which are directly related to the output power.
Ultrasound works by sending ultrasound waves and measuring their reflection. These waves produce heat as they pass through tissues, termed the Thermal Index. Using the Doppler setting further concentrates these waves to a small area, accentuating the energy force produced on the fetus. This heat can burn the fetus if used inappropriately. This is why M-mode is the standard setting for checking the fetus’ heart rate, to try to keep the radiation as low as reasonably achievable. This is recommended by the American Institute of Ultrasound in Medicine (AIUM). Specifically, they recommend that “when an embryo/fetus is detected, it should be measured and cardiac activity recorded by a 2-dimensional video clip or M-mode imaging. Use of spectral Doppler imaging is discouraged.” 6
The World Federation for Ultrasound in Medicine and Biology and International Society of Ultrasound in Obstetric and Gynecology (WFUMB/ISUOG) released a statement on the safe use of doppler ultrasound during early pregnancy scans (before week 14 of gestation) in 2013.5 Among their recommendations, they specify that Pulsed Doppler (spectral, power and color flow imaging) ultrasound should not be used routinely, but may be used for clinical indications such as to define risks for trisomies, which is beyond the scope of the emergency physician. Even in that context, the Thermal Index and exposure time should be kept as low as possible, emphasizing their potential harm.
In summary, the BhCG level should not be used solely to rule ectopic pregnancy in or out, as the discriminatory zones are not sensitive or specific for this diagnosis. Therefore, an ultrasound should be performed on all pregnant patients with symptoms concerning for an ectopic pregnancy. In addition, if a pregnancy is identified, M-mode should be used to detect the fetal heart rate, and Doppler should be avoided due to its potential harm.
1. Doubilet PM, Benson CB. Further evidence against the reliability of the human chorionic gonadotropin discriminatory level. J Ultrasound Med. 2011 Dec;30(12):1637-42
2. Condous G, Kirk E, Lu C, et al. Diagnostic accuracy of varying discriminatory zones for the prediction of ectopic pregnancy in women with a pregnancy of unknown location. Ultrasound Obstet Gynecol. 2005 Dec;26(7):770-5
3. Wang R, Reynolds TA, West HH, et al. Use of a β-hCG discriminatory zone with bedside pelvic ultrasonography Ann Emerg Med. 2011;58:12-20.
4. Hahn SA, Lavonas EJ, Mace SE, et al. Clinical policy: Critical issues in the initial evaluation and management of patients presenting to the emergency department in early pregnancy. Ann Emerg Med. 2012;60:381-390.
5. WFUMB/ISUOG statement on the safe use of Doppler ultrasound during 11-14 week scans (or earlier in pregnancy). Ultrasound Med Biol. 2013 Mar;39(3):373.
6. American Institute of Ultrasound in Medicine, AIUM practice guideline for the performance of obstetric ultrasound examinations. J Ultrasound Med. 2010 Jan;29(1):157-66.