A short cervix is a cervix that measures 25 millimeters or less during the second trimester of pregnancy. The cervix is the narrow lower portion of the uterus that opens into the vaginal canal, and during pregnancy it normally stays long and closed to support the growing baby. When it shortens too early, the risk of preterm birth rises significantly.
How a Short Cervix Is Measured
Cervical length is measured using a transvaginal ultrasound, typically between 18 and 24 weeks of pregnancy. This has become standard screening in many countries. A small ultrasound probe is placed in the vagina to get an accurate image of the cervix, which is more reliable than measuring through the abdomen. The threshold that doctors use is 25 millimeters. Anything at or below that length is considered short and warrants closer monitoring or treatment.
To put that in perspective, a normal cervix during the second trimester is roughly 30 to 40 millimeters long. The shorter the measurement, the higher the concern. Among high-risk women whose cervix measured under 25 mm, 42% delivered before 35 weeks. Even women considered high-risk but whose cervix stayed at 25 mm or above still had a 16% rate of early delivery, which shows that cervical length is one important factor among several.
Why It Happens
The most common cause is previous trauma to the cervix. This includes cervical lacerations during a difficult vaginal delivery (particularly a very fast birth or a prolonged pushing stage), a LEEP procedure to treat abnormal cervical cells, a cone biopsy, or any surgery requiring the cervix to be mechanically opened, such as a dilation and curettage. Each of these can weaken the structural tissue of the cervix, making it more likely to shorten under the weight of a growing pregnancy.
Less commonly, some people are born with a cervix that is structurally weaker. Congenital causes include uterine abnormalities that developed before birth, connective tissue disorders like Ehlers-Danlos syndrome that affect collagen production, and in utero exposure to a drug called diethylstilbestrol (DES) that was prescribed to pregnant women decades ago. People with congenital causes often present with severely shortened cervical lengths.
Symptoms to Watch For
A short cervix often causes no symptoms at all, which is why ultrasound screening matters. Some women notice mild warning signs before a diagnosis, usually before 24 weeks. These can include a sensation of pelvic pressure, a new or unusual backache, mild abdominal cramping, a change in vaginal discharge, or light vaginal bleeding. None of these symptoms is specific to cervical shortening, so they’re easy to dismiss. If you’re experiencing any combination of them during the second trimester, it’s worth mentioning to your provider.
Treatment With Vaginal Progesterone
For women with a singleton pregnancy and a short cervix, vaginal progesterone is one of the primary treatment options. The hormone helps maintain the cervical tissue and reduce the risk of preterm delivery. It’s typically administered as a daily vaginal suppository or gel starting when the short cervix is identified and continuing into the third trimester.
One important distinction: vaginal progesterone appears to work specifically when a short cervix is present. In women with a history of preterm birth but a normal cervical length, the evidence does not show it prevents another early delivery. A meta-analysis found that vaginal progesterone was not associated with a reduction in recurrent preterm birth when no cervical shortening was documented. This means the treatment is targeted, not a blanket preventive measure.
Cervical Cerclage
A cerclage is a stitch placed around the cervix to hold it closed. There are three scenarios in which it’s used. A prophylactic cerclage is placed early in pregnancy for women who have a history of painless cervical dilation leading to second-trimester pregnancy loss. An ultrasound-indicated cerclage is considered when a woman has a history of preterm birth before 34 weeks and her current cervical length drops below 25 mm before 24 weeks. An emergency cerclage is placed when the cervix is already dilating without contractions, infection, or placental problems, in a pregnancy under 24 weeks.
The procedure does improve outcomes. A large Cochrane review found that women who received a cerclage were less likely to deliver before 37, 34, and 28 weeks compared to those who didn’t. Planned cerclages also lead to higher birth weights and later delivery dates than emergency ones, which underscores the value of early screening and identification.
What About Pessaries and Bed Rest?
A cervical pessary is a silicone ring placed around the cervix to provide support, similar in concept to a cerclage but without surgery. It was once considered promising, but a large NIH-supported trial found it performed no better than usual care. Among women who received a pessary, 45.5% experienced preterm birth or related complications, essentially the same rate as the 45.6% in the usual care group. The pessary group also had a higher rate of fetal or infant death (13.3% versus 6.8%), though the usual care group was more likely to receive cerclage, which may have influenced those numbers.
Bed rest and activity restriction are commonly prescribed but lack evidence of benefit. No randomized trials have demonstrated that bed rest, pelvic rest, or stopping work improves outcomes for women with a short cervix. In fact, research found the opposite pattern: preterm birth before 37 weeks was significantly more common among women placed on activity restriction (37%) compared to those who were not (17%). Even after adjusting for other risk factors, the association held. Activity restriction also carries real downsides, including muscle loss, blood clot risk, lost income, and psychological strain. Current evidence does not support prescribing it for cervical shortening.
What Monitoring Looks Like
If you’re diagnosed with a short cervix, you can expect more frequent ultrasounds to track whether your cervical length is stable, continuing to shorten, or showing signs of funneling (where the internal opening begins to widen). Your provider will use these measurements to decide whether to start progesterone, recommend a cerclage, or adjust your care plan. The monitoring window is most critical between 18 and 28 weeks, when cervical changes have the greatest impact on pregnancy outcomes.
Many women with a short cervix carry their pregnancies to term or near-term with appropriate monitoring and treatment. The diagnosis is serious but manageable, and early identification through routine screening is the single most important factor in getting ahead of it.