Universal or selective cervical length screening?
Examining the evidence
Significant advances in neonatology have improved survival and outcomes for infants born at ever-earlier gestational ages, but preterm birth (PTB) is still the leading cause of infant mortality in the United States, contributing to nearly 35% of all infant deaths.1 Of the nearly half-million premature births in the United States annually, approximately 75% follow either spontaneous preterm labor or preterm premature rupture of membranes.2 Prevention of these spontaneous PTBs is the obvious answer to the question of how to decrease our nation’s PTB rate, but this goal has remained elusive until recently.
Recent progress in understanding the pathways and clinical presentations of PTB has created an opportunity to prevent PTB by prolonging pregnancy in women whose pregnancies are complicated by a short cervix. Once believed to be a marker of insufficient cervical strength or integrity, a short cervix in the second trimester is now recognized as evidence of the early onset of parturition. Reduced rates of PTB in women with a short cervix who are treated with progesterone has led to the realization that the initial steps toward PTB begin long before clinical presentation with preterm labor or ruptured membranes, and can be detected by transvaginal ultrasound measurement of cervical length (CL).
Women with a CL below the fifth percentile (corresponding to CL <25 mm) before 20 weeks' gestation have a markedly increased risk of PTB (likelihood ratio: 4.31 in women with no prior PTB and 11.30 in women with a previous early delivery).3 This risk is reduced significantly by progesterone treatment, and in selected cases, by cervical cerclage as well.4-8
Safe and effective treatments are now available for women with a short cervix.4-6 To this end, sonographic measurement of CL has met the standard criteria for a screening test.9 It addresses a significant health problem, is a safe and relatively inexpensive test that is not difficult to perform, and can identify women at risk of PTB who are eligible for effective treatment.4-6,10,11 The major remaining question about cervical ultrasound screening is how to best define or select women who could benefit from this test. Should it be a part of prenatal care for all women, or used selectively in women who meet certain criteria? Current data indicate that transvaginal ultrasonography (TVUS) should be used more widely, but issues of selection, sonographer training, and cost remain.
CL measurements and risk of PTB
The goal of CL measurement in pregnancy is to identify women with the highest risk of PTB.12 CL measurements at 16 to 22 weeks’ gestation form a normal bell-shaped curve, with 27 mm corresponding to the fifth percentile, 30 mm to the 10th percentile, and 45 mm to the 90th percentile.13 The definition of short cervix varies according to the gestational age at cervical measurement (cervix normally shortens after 22 weeks) and the outcome of interest (gestational age of the PTB).10-14 Although a CL measurement of less than 25 mm is reliably associated with an increased risk of PTB, a treatment benefit for progesterone is evident only with a cervical length less than 20 mm in women with no previous PTB.5,6,10-14
Importance of technique
TVUS has several advantages over transabdominal ultrasonography (TAUS). These include a nearly universal ability to image the cervix regardless of maternal body habitus, ability to detect the effect of transabdominal pressure on the length of the cervix, and improved image quality related to the shorter distance between the ultrasound probe and the cervix.
Adherence to standards for obtaining and measuring ultrasound images of the cervix is essential (Figure 1).15 For example, the volume of urine in the maternal bladder has an unpredictable effect on the length of the cervix, and operator technique can affect the quality of the images obtained.16 For this reason, education and credentialing of sonographers and physicians who obtain CL measurements is recommended. The Perinatal Quality Foundation (ie, CLEAR [Cervical Length Education and Review] program) and the Fetal Medicine Foundation both offer online training and credentialing.15,17