Ectopic Pregnancy Basics

An ectopic pregnancy occurs when a fertilized ovum implants outside the uterine cavity. The most common site is the fallopian tube (approximately 95% of cases), specifically the ampullary segment (70%), followed by the isthmic segment (12%), fimbrial end (11%), and interstitial/cornual segment (2-3%). Less common implantation sites include the ovary (3%), cervix (less than 1%), cesarean section scar, and abdominal cavity (less than 1%). Under normal physiology, after fertilization in the ampulla of the fallopian tube, the zygote is propelled toward the uterine cavity by rhythmic contractions of the tubal smooth muscle (peristalsis) and the beating of ciliated epithelial cells lining the tubal lumen. This transport takes approximately 3-4 days, during which the zygote develops from a morula to a blastocyst. Ectopic implantation occurs when this transport mechanism is impaired, most commonly by damage to the tubal epithelium from prior infection (particularly Chlamydia trachomatis and Neisseria gonorrhoeae), previous tubal surgery, or anatomic abnormalities. Once implanted in the fallopian tube, the trophoblast cells of the developing embryo invade the tubal wall, which lacks the thick, distensible myometrium and specialized decidual response...