The majority of ectopic pregnancies (about 95%) are located in which portion of the fallopian tube?

• A. Ampulla
• B. Isthmus
• C. Fimbria
• D. Interstitial

Answer: (A)

The anatomy of the fallopian tube largely determines where an ectopic pregnancy most often implants. The ampulla is the widest and longest segment where fertilization usually occurs and where the embryo spends more time as it travels toward the uterus. If tubal transport is delayed or impaired, the embryo is most likely to implant in this portion because it provides the window of opportunity for implantation to occur in tubal tissue. That’s why the ampulla accounts for the majority of ectopic pregnancies. The other segments are less common sites for implantation: the isthmus is narrow and transports quickly, the fimbria are responsible for catching the ovulated oocyte rather than supporting implantation there, and the interstitial portion lies within the uterine wall and is relatively rare.