The cervix is the lower, narrow part of the uterus that connects to the vagina. It plays a fundamental role in maintaining a pregnancy by remaining long, firm, and closed, acting as a structural barrier for the developing fetus. When the cervix is measured during the mid-trimester and found to be shorter than expected, it is defined as a short cervix. This finding indicates that the structural integrity may be compromised and warrants close medical attention.
Defining a Short Cervix and Diagnosis Thresholds
Cervical length is measured using a Transvaginal Ultrasound (TVUS), the most accurate method for assessing the cervical canal. This specialized ultrasound is typically conducted during the second trimester, usually between 18 and 24 weeks of gestation, often as part of the routine anatomy scan. The length is measured from the internal os (the opening closer to the uterus) to the external os (the opening closer to the vagina).
A cervical length is considered normal when it measures above 30 millimeters (mm). The clinical threshold for defining a short cervix is set at 25 mm or less. This cutoff represents a length shorter than the 10th percentile for this stage of pregnancy and identifies individuals at increased risk for premature delivery. The risk of preterm birth is inversely related to this measurement: the shorter the cervix, the higher the risk.
A particularly short cervix, measuring 15 mm or less, carries a significantly higher risk compared to one measuring between 20 and 25 mm. This shortening is often associated with funneling, where the internal opening of the cervix begins to widen while the external opening remains closed. This opening may form a V-shape or a U-shape, the latter being associated with earlier delivery in high-risk patients.
The Primary Concern: Risk of Preterm Birth
The main concern associated with a short cervix is spontaneous preterm birth, defined as delivery occurring before 37 weeks of gestation. This condition is sometimes referred to as cervical insufficiency, meaning the cervix cannot withstand the pressure of the growing uterus and fetus. When the cervix is too short, it may begin to efface and dilate prematurely, initiating labor.
A short cervix acts as a strong predictive risk factor for preterm birth, particularly for deliveries occurring before 33 to 35 weeks. However, a short cervix is not a guarantee of an early delivery, as many individuals with this finding carry their pregnancy to term. The heightened risk requires proactive management to minimize potential complications for the infant.
Infants born prematurely face health challenges because their organs have not had sufficient time to fully develop. Complications include respiratory distress syndrome, low birth weight, and increased need for admission to the neonatal intensive care unit. The goal of care is to prolong the pregnancy, allowing the fetus more time to mature in the uterus.
Standard Management and Treatment Options
Once a short cervix is identified, the standard approach involves interventions aimed at maintaining the pregnancy and reducing the risk of preterm birth. The primary medical treatment is vaginal progesterone, a naturally occurring hormone that helps keep the uterus quiet and supports cervical integrity. This therapy is typically administered daily as a 200 mg vaginal suppository or gel, and it has been shown to reduce the rate of preterm delivery before 33 weeks of gestation.
Progesterone supplementation is often continued from detection until approximately 36 weeks of pregnancy. The hormone works through anti-inflammatory effects and by decreasing the sensitivity of the uterine muscle to contractions. This non-surgical approach is the first-line treatment for individuals with a short cervix who do not have a prior history of spontaneous preterm birth.
Another common intervention is the cervical cerclage, a minor surgical procedure where a strong stitch is placed around the cervix to reinforce and keep it closed. This procedure is generally reserved for individuals who have a history of prior spontaneous preterm birth and a short cervix (less than 25 mm) in the current pregnancy.
The cerclage is usually placed before 24 weeks of gestation and is removed near term, typically between 36 and 37 weeks. Continuous monitoring with follow-up transvaginal ultrasounds is an important component of the management plan to track any further changes in cervical length.