Why would my doctor say my cervix is incompetent?

Opens up too early. The cervix normally has a very small opening--the os--even during pregnancy. It should remain so until close to delivery at which time passage of the baby through the os is needed. Thus, close to birth time, the cervix begins to dilate from about <1cm to 10. If dilating too early (opens up), there is risk of preterm delivery, hence concerning for incompetent cervix. Hope that helps. Good luck.