Cervical funneling describes a change in the pregnant cervix where the internal opening begins to dilate and shorten. This funnel-like shape appears as the lower part of the uterus opens prematurely, allowing the amniotic membranes to protrude into the cervical canal. This finding indicates cervical insufficiency, a condition where the cervix cannot maintain the pregnancy until term, raising the risk of spontaneous preterm birth. Modern obstetric management offers effective strategies to stabilize the cervix and increase the chance of carrying the pregnancy to term.
Identifying Cervical Funneling
Cervical funneling is primarily discovered through a transvaginal ultrasound (TVUS) examination, the most accurate method for assessing cervical changes during pregnancy. TVUS allows measurement of the functional length and visualization of the internal structure. Funneling is defined as the protrusion of the amniotic sac or membranes into the internal cervical os by more than five millimeters.
The cervix connects the uterus to the vagina, featuring an internal os (closest to the baby) and an external os (closest to the vagina). Funneling occurs when the internal os opens first, creating the characteristic shape while the external os remains closed. Clinicians classify the shape as T, Y, V, or U, with U-shaped funneling indicating a more advanced degree of cervical change.
Funneling is typically assessed in the mid-trimester, most commonly between 16 and 24 weeks of gestation, especially in high-risk patients. It is evaluated alongside the total cervical length; a measurement under 25 millimeters is considered a short cervix. The combination of a short cervix and visible funneling strongly predicts an elevated risk for preterm delivery.
Medical Interventions to Stabilize the Cervix
The definitive mechanical intervention to counteract cervical funneling is a cervical cerclage. This procedure involves placing a strong suture high on the cervix to reinforce its structural integrity and keep the internal os closed. Primary techniques include the McDonald or Shirodkar cerclage, which aim to increase the functional length of the cervix under the strain of the growing pregnancy.
A cerclage may be placed preventatively (history-indicated) based on a patient’s prior second-trimester loss due to cervical insufficiency. Alternatively, it can be ultrasound-indicated, placed after a short cervix (less than 25 mm) or funneling is detected on a mid-trimester scan. If the cervix is visibly dilating with membranes protruding, an emergency or rescue cerclage may be performed to salvage the pregnancy.
Pharmaceutical therapy, alongside physical support, plays a significant role in stabilizing the cervix and preventing preterm labor. Progesterone, a naturally occurring hormone, is commonly administered to patients with a short cervix or visible funneling. This therapy is typically given as a vaginal suppository, gel, or weekly injection, beginning in the mid-trimester and continuing until about 36 weeks.
Progesterone maintains uterine quiescence, keeping the uterine muscle relaxed and less prone to contractions. It also prevents the biochemical process of cervical ripening that leads to shortening and dilation. For many patients, combining cerclage and progesterone therapy offers the highest success rate in prolonging the pregnancy.
Activity Restriction and Non-Surgical Support
Non-surgical management focuses on reducing physical pressure and strain on the compromised cervix. While the term “bed rest” is often used, strict, full-time bed rest is generally not recommended due to risks like blood clot formation and bone demineralization. Instead, patients are usually advised to follow a regimen of modified activity restriction.
Modified activity restriction commonly involves pelvic rest, meaning abstaining from sexual intercourse and avoiding vaginal insertion of objects. Patients are also instructed to avoid heavy lifting, prolonged standing, and strenuous physical activity that increases abdominal pressure. The goal is to minimize downward force on the internal os, allowing the cervix to remain closed.
Another non-surgical option is the cervical pessary, a small, flexible silicone ring inserted into the vagina to encircle the cervix. The device provides mechanical support by changing the angle of the cervix and redistributing pressure away from the internal os. The pessary is less invasive than cerclage, requiring no anesthesia for placement or removal.
Evidence supporting the routine use of the pessary over standard care (progesterone or cerclage) remains under discussion. While some studies suggest a benefit for certain high-risk groups, others have not shown a clear advantage in preventing preterm birth. The decision to use a pessary is highly individualized, often based on a patient’s anatomy or when other interventions are not feasible.
Ongoing Care and Delivery Planning
Once a treatment plan is in place, the pregnancy requires vigilant monitoring to track the cervix’s stability. Serial transvaginal ultrasounds are usually performed every one to two weeks, often until around 28 weeks of gestation, to ensure the intervention is holding. Clinicians monitor for further shortening of the functional cervical length or descent of the amniotic sac.
Patients must be aware of warning signs indicating treatment failure or the onset of labor. These signs include persistent pelvic pressure, vaginal bleeding, leaking fluid, or regular, painful contractions. Immediate medical evaluation is necessary if these symptoms occur, as they signal impending preterm labor or rupture of membranes.
If a patient has an ultrasound-indicated cerclage, the stitch is planned for removal when the baby is considered full-term, typically between 36 and 37 weeks. Suture removal is usually a quick, straightforward outpatient procedure requiring no operating room. Once the cerclage is removed, the body can progress toward labor, allowing for a healthy delivery at or near the expected due date.