Which of the following is the commonest site of ectopic pregnancy?
**Core Concept**
The commonest site of ectopic pregnancy is related to the anatomy of the fallopian tube, specifically the ampulla, where fertilization typically occurs. The fallopian tubes are the narrow tubes that connect the ovaries to the uterus, and their muscular layer can sometimes contract and push the fertilized egg towards the uterine cavity, but in some cases, the egg implants in the tube itself, leading to an ectopic pregnancy.
**Why the Correct Answer is Right**
The ampulla is the widest part of the fallopian tube, and it is the most common site for ectopic pregnancy due to its larger diameter and the presence of cilia that help move the fertilized egg towards the uterus. However, in cases where the cilia are damaged or the fertilized egg does not move properly, it can implant in the ampulla, leading to an ectopic pregnancy. The ampulla's location near the ovary also increases the risk of ectopic pregnancy.
**Why Each Wrong Option is Incorrect**
**Option A:** The isthmus is a narrower section of the fallopian tube, and while it can be a site for ectopic pregnancy, it is less common than the ampulla.
**Option B:** The interstitial portion of the tube is the portion that passes through the uterine wall, and while it can be a site for ectopic pregnancy, it is also less common than the ampulla.
**Option C:** The ovary is not a common site for ectopic pregnancy, as the fertilized egg typically implants in the uterine cavity rather than the ovary.
**Clinical Pearl / High-Yield Fact**
It's essential to note that the risk of ectopic pregnancy increases with factors such as previous ectopic pregnancy, pelvic surgery, and assisted reproductive technology (ART) procedures.
**Correct Answer:** C. The ampulla is the commonest site of ectopic pregnancy.