## Most Common Site of Ectopic Pregnancy **Key Point:** The ampulla is the widest and most distensible segment of the fallopian tube, accounting for approximately 70% of all tubal ectopic pregnancies. ### Anatomical Distribution of Tubal Ectopic Pregnancies | Tubal Segment | Frequency | Key Features | |---|---|---| | **Ampulla** | ~70% | Widest segment; thin muscular wall; most common site | | Isthmus | ~12% | Narrow, muscular; higher rupture risk | | Fimbriae | ~11% | Funnel-shaped; may result in tubal abortion | | Interstitial | ~2–4% | Intramural; rare but carries highest rupture risk; late presentation | **High-Yield:** The ampulla's larger lumen allows the blastocyst to implant and grow longer before rupture occurs. This is why ampullary pregnancies often present with tubal abortion (expulsion through the fimbriae) rather than rupture. ### Why Ampulla Is Most Common 1. **Anatomical factors:** The ampulla is the longest and widest segment of the tube, providing the least resistance to implantation and early growth. 2. **Embryological basis:** The blastocyst naturally lodges here during normal tubal transit before reaching the uterus. 3. **Clinical outcome:** Ampullary pregnancies are more likely to present with chronic ectopic pregnancy or tubal abortion rather than acute rupture. **Clinical Pearl:** A ruptured isthmic pregnancy is more likely to present with acute hemorrhage and shock due to the narrow lumen and thick muscular wall, whereas an ampullary pregnancy may present with lower-grade bleeding and tubal abortion. **Mnemonic:** **AAIF** — Ampulla (70%), Isthmus (12%), Interstitial (2–4%), Fimbriae (11%). The ampulla is the **A**pproach — the first major segment the embryo encounters. [cite:Jeffcoate's Principles of Gynaecology Ch 12]
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