Ectopic Pregnancy: Don't forget the Endomyometrial Mantle!

This week’s image is a ruptured tubal ectopic pregnancy that teaches us an important lesson about the importance of looking at structures in multiple planes to understand three-dimensional relationships within the body.  

The first image is a sagittal section through the pelvis obtained using the intracavitary probe with the indicator towards the ceiling. 

Here we can see a gestational sac, with a fetal pole and a yolk sac.  In this two-dimensional picture, it appears the pregnancy could be in the uterus. Even on the video clip obtained in this plane, it was difficult to distinguish if this pregnancy was inside or outside the uterus.  Free fluid is our only tip off that there is an abnormality. 

The second image is a coronal view of the uterus, which was obtained by rotating the probe counterclockwise 90 degrees (indicator now toward the right shoulder). 

Here we can more clearly see the uterus as a separate structure from the ectopic pregnancy below.

Recall that in our confirmation of intrauterine pregnancy, we measure the endomyometrial mantle (distance from the wall of the gestational sac to the outer wall of the uterus) to exclude an interstitial or cornual ectopic.  In a normal pregnancy, centered within the uterus, this measurement should be >0.8cm.  It is critical that we measure the thinnest wall in two planes to get a true sense of anatomic relationship in 3-D space to rule out an ectopic (especially the often difficult to appreciate corneal or interstitial ectopic pregnancy).  

In this case, the ectopic pregnancy was within the tube, but closely abutting the lateral wall of the uterus.  The close proximity and similar echogenicity of theses structures makes them difficult to distinguish unless both planes are interrogated.  The ectopic pregnancy was identified in this case, but remember that finding the ectopic is not our primary endpoint as emergency providers.  Our primary question is... Is there an intrauterine pregnancy? [Defined as a gestational sac with either a fetal pole or a yolk sac (or both), surrounded by a thick rim of myometrium, with endomyometrial mantle >0.8cm in two planes.] If the answer to this question is no, there is an ectopic pregnancy until proven otherwise.