When you're pregnant, your cervix makes a mucus plug to block the opening to the uterus and keep bacteria out. If you have had most of your cervix removed, like with a trachelectomy, or even had just part of it removed from a LEEP or cone biopsy, what makes the plug?
Well, here's the deal: even with a trachelectomy, the entire cervix is not removed. A little bit of the internal cervix is left and reattached to the vagina.
Remember, the cervix is not a separate body part--it's the lower part of the uterus. It's like a tunnel, so there is an internal side to the tunnel leading into the uterus and an external side that leads out to the vagina.
So there is enough cervical tissue left after surgery to be able to make a plug and keep your baby safe. Your estrogen and progesterone are really high during pregnancy, which stimulates the cervical mucus production.