Your resident presents a case to you: this patient is a 24 year old female with a positive pregnancy test at home and right pelvic pain, mild vaginal bleeding. Your resident shows you the following image. What is the diagnosis?
Answer: ectopic pregnancy
This single still image demonstrates a gestational sac with a clearly visible yolk sac located within. What this scan fails to demonstrate is the location of the gestational sac. You take the resident back into the room and obtain the transabdominal pelvic view below. It is apparent from this scan that the uterus is, in fact, empty.
In the patient not on fertility treatment, we can essentially rule out ectopic pregnancy by ruling in an intrauterine pregnancy.1
However, to accurately determine the location of a pregnancy, multiple views should be obtained.2
The appearance of a thickened hyperemic fallopian tube seen on the first image, a so called ectopic tubal ring, can closely resemble that of myometrium, and is identifiable in about half of ectopic pregnancies on trans-vaginal ultrasound (TVU).3
When scanning for ectopic pregnancy, it is imperative that you obtain and document the sagittal view of the uterus to locate any suspected intrauterine pregnancy (IUP).
The usual evaluation of this patient cohort starts with a transabdominal pelvic ultrasound. If equivocal, a bedside TVU should be obtained. If the TVU remains equivocal, you should consider obtaining a comprehensive radiology-performed TVU.2
Remember that an IUP is defined by an intrauterine gestational sac (GS) either a yolk sac or a fetal pole (FP) within it.
Be careful, sometimes clot within an apparent GS can resemble a FP. Definite signs of a FP are the identification of a heart beat or arm/leg buds.
Although well described in radiology literature as being specific for IUP, the double decidual sign is subjective.2It is therefore the shared opinion of this author that this sign should not be utilized in point of care sonography.
Mukul LV, Teal SB. Current management of ectopic pregnancy. Obstetrics and gynecology clinics of North America. 34(3):403-19, x. 2007. [pubmed]
Dawson M, Mallin M, Introduction to bedside ultrasound volumes 1 + 2. [iTunes]
Carol M. Rumack MD FACR, Stephanie R. Wilson MD, J. William Charboneau MD, Deborah Levine. Diagnostic Ultrasound. Mosby; 2011. [inkling]