Cervical thinning usually occurs in the last month of your pregnancy as your cervix stretches. Your doctor can tell you how far effaced you are, 25% effaced, 50% effaced. This tells you how stretched your cervix has become.
As you approach your due date, the body is preparing itself for labor. Pregnancy hormones soften your cervix and the contractions dilate and thin it. A thinner cervix allows the cervix to dilate more easily for birth. When the cervix is fully opened up, it forms one continuous channel with the uterus through which the baby passes.