Cervical Length
Cervical Length and Preterm Birth: What You Need to Know
What is the cervix?
The cervix is the lower, narrow part of the uterus that connects to the vagina. During pregnancy, the cervix remains closed and firm, helping to keep the fetus safely inside the uterus. As labor approaches, it undergoes physiological changes—thinning (effacement) and opening (dilation)—to allow for childbirth.
Why is cervical length important?
Cervical length is a critical predictor of the risk for preterm birth. A normal cervical length in the second trimester, varies considerably amongst women and can measure typically between 35 and 45 millimeters. When the cervix shortens prematurely, the likelihood of spontaneous preterm birth increases, especially before 37 weeks of gestation.
What does “short cervix” mean?
A “short cervix” is defined as a cervical length less than 25 millimeters before the 24th week of pregnancy. This condition is often asymptomatic but significantly increases the risk of preterm birth. The shorter the cervix, the higher the risk:
- Length <25 mm: ~18% risk
- Length <20 mm: ~25% risk
- Length <15 mm: ~50% risk
How is it diagnosed?
The most accurate method for measuring cervical length is transvaginal ultrasound. This is typically performed at 13 weeks (typically during the nuchal translucency scan) between 16 and 24 weeks of gestation (typically during the anomaly scan). In women with a history of preterm birth, regular monitoring every 1–2 weeks during this period is recommended.
What causes a short cervix?
Cervical insufficiency may be congenital or acquired. Congenital causes include anatomical uterine or cervical abnormalities. Acquired factors may involve previous cervical surgeries, such as cone biopsy (LLETZ), or cervical trauma from labor or instrumentation.
What are the potential consequences?
Cervical insufficiency is associated with an increased risk of second-trimester pregnancy loss or spontaneous preterm birth. Preterm birth remains a major cause of neonatal morbidity and mortality, and it may lead to long-term complications such as developmental and neurological impairments.
How is it managed?
Management depends on cervical length and patient history:
- Progesterone supplementation: For cervical lengths below 25 mm, 400 mg of vaginal progesterone may be administered to help maintain cervical integrity and reduce inflammation.
- Cervical cerclage: In cases of cervical length <15 mm before 24 weeks, or in women with a history of second-trimester pregnancy losses due to cervical insufficiency, a cerclage (a surgical stitch placed around the cervix) may be considered.
- Cervical pessary: In selected cases, a silicone pessary may be placed in the vagina to support the cervix and reduce mechanical strain, potentially lowering the risk of preterm delivery. However, this method has not been proven to offer any substantial benefit.
Conclusion
Early detection and appropriate management of a short cervix are vital in preventing preterm birth. Routine cervical length screening via transvaginal ultrasound and individualized treatment strategies can significantly improve pregnancy outcomes and fetal well-being.