The cervix is the narrow, lower part of the uterus that should remain closed (or close to it) until it’s time to give birth. However, it isn’t only the dilation — or how open the cervix is — that matters when it comes to worrying about giving birth prematurely. The length of the cervix can also come in to play, and having a short cervix can increase a woman’s chance of delivering too soon.
The average length of a woman’s cervix is about 3 to 4 centimeters long. Traditionally, a short cervix has been defined as one that is less than 2.5 centimeters long, and in general the shorter the cervix is, the higher the risk of having a premature baby.
The length of the cervix can be measured in two ways. It can be seen on ultrasound (and the most accurate kind of ultrasound for this is a transvaginal ultrasound) or it can be felt during a vaginal exam. Since ultrasound is more accurate, many doctors and midwives will order this test if during their exam they felt the cervix to be short, not only to confirm their findings but also to measure exactly how long it is.
You may then wonder if all women should have the length of their cervix measured early in pregnancy to know if theirs might be short, so something can be done to prevent going into labor prematurely. As of now, the American Congress of Obstetricians and Gynecologists does not mandate this practice; it says measurement for all women can be considered, but that more research is needed. They do say that if doctors do choose to screen everyone, strict protocols for measuring the cervix and following-up with treatments should be adhered to in order to avoid unnecessary tests, treatments, and worry.
To avoid overtreatment, most practitioners will only screen a woman for a short cervix if she is high risk for a preterm delivery, or if there is concern that she may be experiencing premature labor. Some things that would put her in the high risk category include a previous preterm birth, a previous diagnosis of a short cervix, vaginal bleeding in the pregnancy, a history of certain surgeries on the cervix, and being pregnant with twins or triplets.
If a short cervix is diagnosed, various treatments may be used depending on a few other factors, such as if the woman is having contractions or if she has a history of delivering preterm or not. Progesterone placed in the vagina daily (usually in a pill or gel form) has been shown to decrease the chance of giving birth early in women with a short cervix, so this may be prescribed.
For other women, a stitch called a cerclage may be placed around the cervix to help hold it closed. This is a quick surgical procedure that is usually done if additional criteria, other than just a short cervix, are met (such as if a woman has also given birth before 34 weeks in another pregnancy). This stitch is then kept in place until the woman is close to term.
Many women who are found to have a short cervix with no other risk factors or issues go on to have healthy, full term pregnancies. With additional testing (such as follow-up ultrasounds to see if the cervix shortens further), counseling, and some of the treatments described here, good outcomes can be achieved.
Reviewed by Jennifer Lincoln, January 2019
- A short cervix is usually defined as being less than 2.5 centimeters in length.
- Cervical length can be measured by a vaginal exam or transvaginal ultrasound.
- Women who are found to have a short cervix are at an increased risk for preterm birth.