Clinical Review

The short cervix and preterm birth: 8 key questions and evidence-based answers

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An expert review of screening, identification, and management for both nulliparous women and those with a history of spontaneous preterm birth


 

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CASE: NULLIPAROUS WOMAN WITH A SHORT CERVIX
Your ultrasound technician telephones to report that your 32-year-old nulliparous patient, who is currently at 20 weeks’ gestation, was incidentally found to have a short cervix (18 mm) at the time of her routine fetal anatomy survey.

How do you proceed? And how do you counsel the patient? What interventions might reduce the risk of preterm birth (PTB)? Would your recommendations change if she had a history of PTB or was carrying twins?

Preterm birth, defined as delivery prior to 37 weeks’ gestation, is the leading cause of neonatal morbidity and mortality in the United States. The rate of PTB peaked at 12.8% in 2006 and has slowly declined since but remains unacceptably high at 11.5%.1 Most PTBs are spontaneous, arising from the onset of labor or from preterm premature rupture of membranes. Regrettably, tocolytics remain largely ­ineffective once the process of preterm parturition has begun.

Ideally, women at highest risk for PTB could be identified so that additional screening and interventions could be initiated. Few prognostic tests are available to predict which women will deliver preterm. Generally, the greatest risk factor for spontaneous PTB is a history of spontaneous PTB.2,3 However, women with such a history account for only 10% of all births before 34 weeks’ gestation.

The appearance and length of the cervix during the second trimester appears to be an even better predictor of spontaneous PTB than history alone (FIGURE 1).4,5 For example, in one study of unselected pregnant women at 22 to 24 weeks’ gestation, only 1.7% had a cervical length less than 15 mm, but they accounted for 58% of births before 32 weeks.6 The shorter the cervix, the greater the risk of spontaneous PTB.7 The presence of a short cervix is even more ominous in a woman with a history of spontaneous PTB.8



Optimal pregnancy management after detection of a short cervix remains somewhat unclear and varies, based on the rest of the patient’s clinical picture and obstetric history.

In this article, I address 8 critical questions about diagnosis and management of the short cervix in the second trimester and offer evidence-based answers for clinical practice.

1. How is a short cervix defined?
A cervical length below the 10th centile for gestational age is considered “short.” At 18 to 24 weeks’ gestation, the 10th centile corresponds to a cervical length of less than 25 mm.9

The cervix undergoes physiologic shortening that begins at 28 to 30 weeks of gestation. At 32 weeks, the 50th centile for cervical length is 25 mm. Therefore, cervical-length measurements that appear moderately short between 28 and 32 weeks and beyond are of limited clinical utility, and the clinician should incorporate gestational age into prematurity risk assessment.7

2. Who should be screened?
The question of whether universal cervical-length assessment should be performed is controversial. Several decision analyses in recent years suggest that universal sonographic screening for a short cervix is cost-effective.10,11 Overall, however, the effectiveness of universal cervical-length screening remains clinically understudied, and it is difficult to draw conclusions from decision analyses. Moreover, there is considerable concern about resources and feasibility of implementing universal vaginal cervical-length assessment, as well as significant disagreement about the accuracy of transabdominal cervical-length assessment in the detection of a short cervix.

Transabdominal ultrasound may overestimate cervical length by as much as 10 to 15 mm. One recent study demonstrated that, using a transabdominal cutoff of 30 mm, the sensitivity of detecting a transvaginal cervical length of less than 20 mm was 90%; if the cutoff was increased to 35 mm, sensitivity increased to 100%.12

A collaborative practice guideline on obstetric ultrasound from the American College of Radiology, American Institute of Ultrasound in Medicine, American College of Obstetricians and Gynecologists, and the Society of Radiologists in Ultrasound recommends that the maternal cervix be examined “as clinically appropriate when technically feasible” during a standard second- or third-trimester ultrasound examination (FIGURE 2).13 The guideline also states that transvaginal or transperineal ultrasound may be considered if the cervix appears shortened or cannot be adequately visualized during the transabdominal ultrasound. However, no specific protocols are suggested.


Given the uncertainty, it is recommended that each practice or ultrasound unit adopt a standard protocol for cervical-length assessment during pregnancy. This protocol can entail either routine abdominal or vaginal assessment of the cervix, or a combination of abdominal and vaginal assessment. Clinical risk factors can be used to help stratify low-risk women when abdominal cervical-length assessment is the initial approach to evaluation.

In my practice, all women undergo ­cervical-length assessment at the time of the routine anatomy survey (18–22 weeks). Those who are at low risk for PTB are screened initially with transabdominal ultrasound, and a transvaginal examination is performed if the cervix cannot be seen or appears to be less than 30 mm in length.

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