What is a major risk factor for ectopic pregnancy?

A major risk factor for ectopic pregnancy is previous pelvic or abdominal surgery. When a woman has undergone such procedures, there may be scar tissue formation or adhesions within the pelvic area, which can disrupt the normal pathway for the fertilized egg to travel from the ovary to the uterus. This disruption can lead to the fertilized egg implanting outside the uterus, commonly in the fallopian tubes, resulting in an ectopic pregnancy.

While multiple gestations, advanced maternal age, and smoking can influence pregnancy outcomes in various ways, they do not directly impact the physical environment and pathways within the reproductive system as past surgeries can. It's the structural changes from surgery that primarily increase the likelihood of an ectopic pregnancy.