From the Guidelines
A short cervix in the second trimester is strongly associated with an increased risk of preterm birth, with approximately 25-30% of women who deliver preterm having had a cervical length less than 25mm when measured between 18-24 weeks gestation. The relationship between cervical length and preterm birth risk is inverse, with shorter cervix lengths corresponding to higher risks of preterm delivery 1. For women with a cervical length less than 15mm, the risk of preterm birth before 32 weeks increases to about 50%. This association exists because cervical shortening is often an early sign of the physiological process leading to preterm labor, reflecting decreased structural integrity of the cervix.
Key Findings
- Cervical length screening via transvaginal ultrasound during the second trimester (typically at 18-24 weeks) can identify women at higher risk who might benefit from interventions such as vaginal progesterone (200mg daily until 36 weeks), which has been shown to reduce preterm birth risk by approximately 40% in women with a short cervix 1.
- The Society for Maternal-Fetal Medicine recommends using a midtrimester cervical length of 25 mm to diagnose a short cervix in individuals with a singleton gestation and no previous history of spontaneous preterm birth (GRADE 1C) 1.
- Asymptomatic individuals with a singleton gestation and a transvaginal cervical length of 20 mm diagnosed before 24 weeks of gestation should be prescribed vaginal progesterone to reduce the risk of preterm birth (GRADE 1A) 1.
Recommendations
- Cervical length screening is crucial for identifying women at risk of preterm birth, and interventions such as vaginal progesterone should be considered for those with a short cervix.
- Cervical cerclage may be considered for women with a history of preterm birth and a short cervix, but its use is not recommended for women without a history of preterm birth who have a sonographic short cervix (10-25 mm) in the absence of cervical dilation (GRADE 1B) 1.
- Cervical pessary should not be placed for the prevention of preterm birth in individuals with a singleton gestation and a short cervix due to potential safety concerns and lack of consistent data demonstrating benefits (GRADE 1B) 1.
From the Research
Likelihood of Short Cervix in Second Trimester for Preterm Delivery
- The likelihood of a short cervix in the second trimester for a patient who delivers preterm can be associated with an increased risk of spontaneous preterm birth, as indicated by studies 2, 3, 4.
- A short cervical length (≤25 mm) at 23-28 weeks' gestation is an independent risk factor for spontaneous preterm birth, with the risk increasing as cervical length decreases 2.
- Women with a history of short cervix in their first pregnancy, but who delivered at term, are at increased risk of having a short cervix and spontaneous preterm birth in their second pregnancy 3.
Predictive Factors for Preterm Birth
- A short cervix in the second trimester can be predictive of preterm birth, with the probability of short cervix between 19 and 24 weeks being greater for those with shorter first-trimester cervical length 5.
- Serial measurements of cervical length at up to 23 weeks 6 days significantly improve the prediction of spontaneous preterm birth in high-risk women 4.
Management and Treatment
- Antenatal corticosteroids may not be beneficial for asymptomatic women with a short cervix unless delivery is expected to occur within a specific time frame, as the risk of delivery within 1 or 2 weeks is highly unlikely 6.
- Management decisions, such as the timing of administration of antenatal corticosteroids, may need to be delayed until additional indications are present 2.