A short cervix during pregnancy is a medical diagnosis that can raise questions about the safety of the developing fetus. The cervix, the lower, cylindrical part of the uterus, plays a fundamental role in pregnancy by remaining long, firm, and closed throughout gestation. When labor begins, the cervix softens, shortens, and opens to allow the baby to pass into the vagina. A short cervix suggests this natural process of shortening and opening may be beginning too early.
Defining and Measuring Cervical Length
The identification of a short cervix relies on a precise measurement taken during the mid-trimester of pregnancy, most often between 18 and 24 weeks of gestation. This period is when the measurement is most predictive of later outcomes.
The standard and most reliable method for this assessment is a transvaginal ultrasound. This involves inserting a narrow probe into the vagina, which allows for a clear and accurate image of the entire length of the cervix. While an abdominal ultrasound may be used, the transvaginal approach is the gold standard for this specific measurement.
In a typical pregnancy, the cervix measures between 30 and 50 millimeters (3 to 5 cm) long. A cervix is medically defined as “short” if it measures 25 millimeters (2.5 cm) or less before 24 weeks of pregnancy. This cutoff value identifies pregnancies that may benefit from intervention, though cervical length is a dynamic measurement that can change over time.
Understanding the Preterm Birth Risk
A diagnosis of a short cervix is significant because it is strongly linked to an increased risk of spontaneous preterm birth. Preterm birth is defined as any delivery that occurs before 37 full weeks of pregnancy. The risk of a baby being born early rises as the measured cervical length decreases.
The underlying concern is that a short cervix may not be structurally strong enough to sustain the increasing weight and pressure of the growing pregnancy. This premature shortening and potential opening, sometimes referred to as cervical insufficiency, can lead to the onset of labor well before the due date. Preterm birth is the leading cause of newborn health problems and mortality, with earlier deliveries carrying the highest risks.
For women with a short cervix (25 mm or less), the risk of delivering before 34 weeks is substantially elevated compared to those with a longer cervix. However, most women (about 75%) found to have a shortened cervix do not ultimately deliver prematurely. The measurement serves as a powerful predictive tool, allowing medical professionals to identify and manage the elevated risk proactively.
Evidence-Based Management Options
When a short cervix is identified, management focuses on preventing spontaneous preterm birth through evidence-based interventions, such as the use of vaginal progesterone.
Vaginal progesterone is a naturally occurring hormone thought to help maintain the pregnancy by reducing uterine contractions and preventing further shortening. It is typically administered daily as a capsule or gel inserted into the vagina and continued until around 36 or 37 weeks of gestation. Studies show this therapy can significantly reduce the rate of preterm birth before 33 weeks in women with a short cervix. The local delivery of the hormone provides the most direct action on the cervical tissue.
For cases involving an extremely short cervix or a history of previous early deliveries, a surgical procedure called a cervical cerclage may be recommended. This procedure involves placing a strong stitch, like a purse-string suture, around the cervix to reinforce its structure and keep it closed. The cerclage is usually placed before 24 weeks of pregnancy and is removed around 36 or 37 weeks, allowing for a vaginal delivery.
An ultrasound-indicated cerclage is considered for women who have had a prior spontaneous preterm birth and whose cervical length measures less than 25 mm. The decision between progesterone and cerclage is highly individualized, depending on the exact cervical length and the woman’s history of prior pregnancies. These interventions, along with increased prenatal monitoring, are designed to improve outcomes by extending the length of the pregnancy.