The cervix is the cylindrical mouth of the uterus, separating it from the vagina. Normally, the cervix is at least 3-4 cm long (about 1 1/2 inches), and its length in pregnancy strengthens the protection of the fetus. Toward the due date, the cervix normally shortens significantly (effacement) as the body prepares for labor and delivery.
Studies have shown that cervical measurements done by transvaginal ultrasound between 18 and 28 weeks can predict the risk of premature delivery. The shorter the cervix, the more likely it is that there is a risk for premature labor and delivery.
In one study, published in JAMA, Dr. Owen and researchers found that women whose cervixes had shortened to less than 25 mm in length by the time of the first ultrasound evaluation at or after 16 weeks of pregnancy were 3.3 times more likely to give birth prematurely than were the other women in the study. Women whose cervixes had not shortened until later ultrasound evaluations were also more likely to give birth prematurely.