Clinical meaning
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 of the uterus. The tubal wall has only a thin layer of smooth muscle and limited blood supply compared to the endometrium. As the embryo grows, the expanding gestational sac stretches the tubal wall, causing ischemia and necrosis of the surrounding tissue. The trophoblast invasion erodes into tubal blood vessels, which initially may cause intermittent vaginal bleeding (often described as spotting or dark brown discharge, distinct from normal menstrual flow). Without intervention, the growing ectopic pregnancy will eventually rupture the fallopian tube, typically between 6-10 weeks of gestation. Tubal rupture results in hemorrhage into the peritoneal cavity (hemoperitoneum) that can be rapid and massive because the tubal arteries branch directly from the uterine artery system. Hemorrhage from a ruptured ectopic pregnancy can exceed 500-1500 mL within minutes, leading to hemorrhagic shock. Human chorionic gonadotropin (hCG) is produced by the trophoblast cells and is the primary hormone used for diagnostic monitoring. In a normal intrauterine pregnancy, serum beta-hCG levels double approximately every 48-72 hours during the first 8 weeks. In ectopic pregnancy, hCG levels typically rise more slowly (less than 53% increase in 48 hours) or plateau, reflecting impaired trophoblast growth. A discriminatory level of beta-hCG (typically 1500-2000 mIU/mL for transvaginal ultrasound) is the threshold above which a gestational sac should be visible in the uterus; absence of an intrauterine gestational sac above this level is highly suspicious for ectopic pregnancy. Cullen sign (periumbilical ecchymosis) indicates blood tracking along the falciform ligament to the umbilical area and suggests significant intraperitoneal hemorrhage from a ruptured ectopic pregnancy.