Which of the following is a pathognomonic feature of ectopic pregnancy?

Nearly 95 percent of ectopic pregnancies are implanted in the various segments of the fallopian tubes. Of these, most are ampullary implantations. The remaining 5 percent implant in the ovary, peritoneal cavity, or within the cervix. Beta hCG in the urine and serum, empty uterus, adnexal mass, decidual casts and free fluid in peritoneal cavity is pathognomonic of ectopic pregnancy. A trilaminar endometrial pattern is unique for diagnosis of ectopic pregnancy-its specificity is 94 percent, but with a sensitivity of only 38 percent. A decidual cyst is identified as an anechoic area lying within the endometrium but remote from the canal and often at the endometrial-myometrial border. This finding represents early decidual breakdown and precedes decidual cast formation. Also Know: All pregnancies induce an endometrial decidual reaction, and sloughing of the decidua can create an intracavitary fluid collection called a pseudogestational sac, or pseudosac. When a pseudosac is noted, the risk of ectopic pregnancy is increased. Ref: Cunningham F.G., Leveno K.J., Bloom S.L., Hauth J.C., Rouse D.J., Spong C.Y. (2010). Chapter 10. Ectopic Pregnancy. In F.G. Cunningham, K.J. Leveno, S.L. Bloom, J.C. Hauth, D.J. Rouse, C.Y. Spong (Eds), Williams Obstetrics, 23e.