Cervical length at approximately 24 weeks into a pregnancy is an excellent predictor of a pregnant woman’s risk of preterm birth. Cervix length is most accurately measured by transvaginal ultrasound, which your doctor may not consider doing unless you specifically request it.
The average cervix length is 4 to 5 cm, and it is expected to shorten as you get further along into your pregnancy.
One study found that at 24 weeks gestation, the average cervix length is 3.5 cm. When cervical length is less than 2.2 cm, women face a 20 percent probability of preterm delivery.
Another study found that when the cervical length measures 1.5 cm or less, the risk of spontaneous preterm birth is almost 50 percent.
Most doctors will schedule women for a transabdominal ultrasound around 20 weeks. Ask the sonographer to take note of your cervical length at that time, and write it down for yourself as well. If the length of the cervix is below 4 cm, ask the sonographer to do a transvaginal ultrasound to get a more accurate measurement. It is standard to take the measurement three times over the course of several minutes. If the length is below 4 cm and you experience ANY signs of preterm labor in the weeks that follow, request a transvaginal ultrasound so a current measurement can be compared to the previous measurement.
Please click here now to learn why a transvaginal ultrasound is much more accurate than a manual exam.