Tips & Tricks for Ultrasound Evaluation of a Blighted Ovum
01. Always begin your pelvic ultrasound with a full bladder and a transabdominal scan to map out the important organs and structures. A full bladder provides an excellent acoustic window. Remember that everyone’s anatomy is different and once you start performing the intracavitary portion of the exam, your view is more limited because of the confined space.
02. On your transabdominal scan, determine how the uterus is lying and determine if there is any obvious free fluid or intrauterine findings.
03. To perform the endovaginal portion of the pelvic ultrasound, start by covering the intracavitary transducer with a gel filled sheath and apply a copious amount of clean gel to the outside of the sheath.
04. Insert and advance the transducer with the indicator marker pointing anteriorly towards the patient’s pubic symphysis. Alternatively, you may let the patient insert the probe herself as this method is often less uncomfortable.
05. Scan through the uterus in this sagittal plane, noting the presence or absence of a gestational sac, yolk sac, fetal pole, fetal heartbeat, etc. If the patient has a positive urine or pregnancy test, and the uterus appears empty, an ectopic pregnancy should be suspected.
06. If you see a gestational sac, carefully evaluate the size and shape of the sac, and correlate the size of the sac to the quantitative hCG level and date of the patient’s LMP.
07. When the gestational sac is larger than 10 mm and no yolk sac is identified, it is likely that the patient has a blighted ovum or anembyonic pregnancy.
08. If you see a yolk sac that is larger then 7 mm but you do not see a developing fetal pole, this also suggests a nonviable intrauterine pregnancy.
09. Remember that the fetal pole is typically seen on transvaginal ultrasound at approximately 5-6 weeks gestation. You should see a fetal pole when the gestational sac is >18 mm on transvaginal ultrasound or >25 mm on transabdominal ultrasound.
10. At times, it is difficult to determine if the sac you are seeing is a true gestational sac, or a pseudogestational sac associated with an ectopic pregnancy. To determine the difference, look at the location of the sac. A gestational sac will be seen within the decidua, whereas pseudogestational sacs are usually in the endometrial canal. When color Doppler is applied over a gestational sac, it will be “warm” and highlight with color. A pseudogestational sac is usually “cold” and no color is visualized on color Doppler.
11. Rememeber, the mean diameter of the gestational sac +42 should = the gestational age of the fetus in days. Use that number to help determine if you see the expected fetal structures for that size of the gestational sac.
12. Patients with a suspected blighted ovum warrant an OB/Gyn consultation in the ED or very close outpatient OB/Gyn follow-up. It is important that patients undergo a follow-up ultrasound with a high-resolution ultrasound machine to ensure that a small yolk sac or fetal pole is not overlooked.
Brady Pregerson manages a free on-line EM Ultrasound Image Library and is the editor of the Emergency Medicine Pocketbook series. For more information visit EMresource.ORG.
Teresa S. Wu is the Associate Residency Director, and Director of Ultrasound and Simulation Programs and Fellowships, for the Maricopa Emergency Medicine Program in Phoenix, Arizona.