What is the principle of emergency contraception?

Patient's question:

Today I learned about the principle of emergency contraception: it prevents the implantation of a fertilized egg. Suddenly, I thought: isn't that ectopic pregnancy? It's so scary! Can you explain it to me? The help I received: I hope to get an answer

Doctor's answer:

The situation is understood: Emergency contraception pills are based on progesterone as their active ingredient. Their primary mechanism is to rapidly transform the endometrium into the secretory phase and enhance the viscosity of cervical mucus, reducing tubal motility. This series of mechanisms can obstruct sperm penetration through the cervix and fertilization with the oocyte, causing the endometrium to prematurely enter the secretory phase, making it unsuitable for the fertilized egg to implant. Reduced tubal motility also hinders the rapid movement of the oocyte, ultimately "blocking the implantation of the fertilized egg."
Recommendation: "Blocking the implantation of the fertilized egg" is a common term. Emergency contraception pills affect sperm-oocyte fusion through a series of steps. If fertilization occurs, the endometrium is also altered to make it unsuitable for implantation. Therefore, there is no need to worry—it is not the same as an ectopic pregnancy. It does not mechanically block the fertilized egg from entering the uterine cavity but rather prevents successful implantation. The full term is "blocking the implantation of the fertilized egg."