Preterm birth, defined as delivery before 37 weeks of pregnancy, represents a significant global health concern. The early arrival of a baby can lead to substantial health challenges, including issues with breathing, feeding, and developmental delays. Understanding the potential risk factors for this outcome is a major focus of modern prenatal care. A short cervix is one such risk factor that medical professionals closely monitor during pregnancy. The cervix is the lower part of the uterus that connects to the vagina. Its primary function is to remain long, firm, and closed to protect the pregnancy. When the cervix begins to shorten prematurely, it signals an increased chance of early delivery.
What Defines a Short Cervix
The cervix is typically a firm structure measuring about 30 to 50 millimeters (3 to 5 centimeters) long during a healthy second-trimester pregnancy. A short cervix is generally diagnosed when this measurement is less than or equal to 25 millimeters (about 1 inch) before 24 weeks of gestation. This shortening indicates a structural change that can compromise the cervix’s ability to remain closed.
This measurement is most reliably taken using a transvaginal ultrasound, where a small probe is gently inserted into the vagina. The transvaginal approach provides a clearer image and a more precise measurement than a standard abdominal ultrasound. Discovering a short cervix is important because it is a powerful predictor of spontaneous preterm birth risk.
How Progesterone Affects the Uterus
Progesterone is a naturally occurring steroid hormone that plays a foundational role in maintaining a healthy pregnancy. It is often referred to as the “hormone of pregnancy” because its presence helps keep the uterus in a quiet state throughout gestation. The hormone is initially produced by the ovaries and later by the placenta.
When administered therapeutically, progesterone works through several biological mechanisms to help prevent premature cervical change. One of its main actions is a calming or “tocolytic” effect on the myometrium, the muscular wall of the uterus. By reducing the excitability and contractility of these muscles, progesterone helps suppress premature uterine contractions that could lead to labor.
The hormone also directly influences the integrity and structure of the cervix itself. Progesterone is thought to exert an anti-inflammatory effect, which is important because inflammation is a known trigger for cervical ripening and premature shortening. Furthermore, it may modulate the composition of the cervical extracellular matrix, the collagen-rich tissue that gives the cervix its strength. By stabilizing the cervical tissue and dampening the muscular activity of the uterus, the hormone supports the cervix in maintaining its length until term.
Clinical Evidence and Treatment Routes
The effectiveness of progesterone for managing a short cervix is strongly supported by clinical research and is now a standard practice in obstetrics. Studies, including major randomized controlled trials, have shown that administering vaginal progesterone can significantly reduce the rate of preterm birth in women identified with a short cervix. This intervention is associated with a reduction in the risk of delivery before 33 weeks of gestation.
The observed reduction in preterm birth typically falls within a range of 30% to 45% for women with a short cervix who receive the treatment. This preventative measure helps extend the pregnancy and is associated with improved outcomes for the newborn, such as a lower incidence of respiratory distress syndrome and a decreased need for neonatal intensive care unit admission. The benefit is seen in women with a singleton pregnancy and an incidentally discovered short cervix.
The standard route of administration for this specific diagnosis is vaginal progesterone, typically delivered as a suppository, gel, or capsule inserted daily. This method is preferred because it delivers a high concentration of the hormone directly to the cervix and lower uterine segment, where it can exert its local effects. Injectable progesterone, known as 17-alpha hydroxyprogesterone caproate (17-OHPC or 17P), is sometimes used for a history of prior spontaneous preterm birth, but it is not the recommended treatment for an isolated finding of a short cervix. Trials have not shown a benefit for 17P in women whose only risk factor is a short cervix measurement. Therefore, vaginal progesterone is the established, evidence-based treatment route.
Eligibility and Safety Profile
The treatment window for progesterone therapy is defined by specific eligibility criteria. Treatment is generally initiated after a short cervix is diagnosed, typically between 18 and 24 weeks of gestation, and continued daily until approximately 36 or 37 weeks of pregnancy. This intervention is generally recommended for women carrying a singleton pregnancy, as evidence for its effectiveness in twin or other multiple gestations remains limited. The aim is to begin treatment before the cervical shortening becomes too advanced to respond effectively.
Progesterone treatment for a short cervix has an overall favorable safety profile and is well-tolerated by most patients. Common side effects are usually minor and localized, primarily involving the application site. Patients may experience some vaginal discharge or local irritation due to the suppository or gel formulation. Crucially, extensive studies have not found a significant increase in adverse fetal outcomes or congenital anomalies associated with the use of vaginal progesterone. This established safety record makes it a valuable and widely recommended treatment option for eligible patients.