Role of Cervical Length Assessment in Routine Obstetric Care
Transvaginal ultrasound assessment of cervical length is an essential component of obstetric care for identifying patients at risk for preterm birth, with specific indications based on pregnancy type and risk factors. 1
Cervical Length Assessment in Singleton Pregnancies
Low-Risk Patients (No History of Preterm Birth)
- Transabdominal cervical assessment is a standard component of second and third trimester ultrasound evaluation 1
- If transabdominal assessment is inadequate or suspicious for abnormality, transvaginal or transperineal ultrasound is preferred 1
- A cervical length ≤25 mm before 24 weeks is considered short in singleton gestations 1
- Management recommendations for short cervix:
- Cervical length ≤20 mm: Vaginal progesterone is strongly recommended 1
- Cervical length 21-25 mm: Vaginal progesterone may be considered based on shared decision-making 1
- Cerclage is NOT recommended for incidental short cervix without prior risk factors 2
- Pessary is NOT recommended for prevention of preterm birth in singleton gestations with short cervix 1
- 17-alpha hydroxyprogesterone caproate is NOT recommended for treatment of short cervix 1
High-Risk Patients (History of Preterm Birth)
- Transvaginal ultrasound cervical screening is recommended 1
- Identification of short cervical length allows for appropriate interventions that can decrease preterm birth rates 1
- Interventions may include cerclage placement, progesterone administration, or pessary placement 1
Cervical Length Assessment in Multiple Gestations
- In twin pregnancies, cervical length measuring <1.5 cm between 15-24 weeks is predictive of preterm labor 1
- Routine transvaginal cervical length screening is not recommended after 25-26 weeks' gestational age in multiple gestations 1
- Monochorionic diamniotic twin pregnancies have significantly lower mean cervical length compared to dichorionic pregnancies (32.8 mm vs 34.9 mm) 1
- Monochorionic twins have higher incidence of spontaneous preterm birth (53.1% vs 44.9%) 1
- Progesterone, pessary, or cerclage are NOT recommended for treatment of cervical shortening in twin gestations outside clinical trials 1
Technical Considerations for Cervical Assessment
Proper Measurement Technique
- All cervical length measurements used to guide therapeutic recommendations should be performed using a transvaginal approach 1
- Measurements should follow standardized procedures as described by organizations such as the Perinatal Quality Foundation or the Fetal Medicine Foundation 1, 2
- Transvaginal ultrasound is the gold standard and reference method for cervical length assessment 1, 2
Alternative Approaches
- Transperineal ultrasound is an acceptable alternative when transvaginal assessment is not possible due to patient preference or clinical contraindications 1
- Transperineal approach has shown good correlation with transvaginal measurements, particularly after 20 weeks' gestation 1
- Inadequate visualization with transperineal approach occurs in up to 22% of patients 1
Clinical Pitfalls and Considerations
- Transabdominal assessment is less reliable than transvaginal and transperineal approaches 1
- Transperineal measurements may be statistically shorter between 14-20 weeks' gestation compared to transvaginal measurements 1
- When short cervical lengths are obtained transperineally in early second trimester, transvaginal imaging should be performed for confirmation 1
- Cervical length is not normally distributed at a given gestational age, requiring appropriate statistical models for interpretation 3
- Risk factors for short cervix include low body mass index (RR=3.5), previous fetal losses between 16-23 weeks (RR=33.1), and previous spontaneous preterm deliveries between 24-32 weeks (RR=14.1) 4
By incorporating cervical length assessment into routine obstetric care, clinicians can better identify patients at risk for preterm birth and implement appropriate interventions to improve outcomes.