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ORIGINAL RESEARCH |
From the *Department of Obstetrics and Gynecology, Poissy-Saint Germain Hospital, Versailles-St. Quentin University; and the Department of Biostatistics, Saint-Louis Hospital, Paris VII University, Erm321 INSERM, Poissy Cedex, France.
Address reprint requests to: Patrick Rozenberg, MD, Centre Hospitalier Poissy-Saint-Germain, Rue du Champ Gaillard 78303 Poissy Cedex, France; e-mail: prozenberg{at}chi-poissy-st-germain.fr.
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ABSTRACT |
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METHODS: This was a prospective study of patients admitted and treated for uterine contractions at 24+0 to 33+6 weeks of gestation with a cervical length of 26 mm or less by transvaginal ultrasonography on admission. Intravenous tocolysis was stopped when delivery was delayed by 48 hours, and cervical length was remeasured before discharge. The primary outcome was preterm delivery, defined by a delivery before 37 weeks. Predictive analysis was based on logistic models, with estimated odds ratios and 95% confidence interval.
RESULTS: One hundred and nine patients were included in the study. The median (first, third quartile) cervical length on admission was 18 (13, 22) mm. The median (first, third quartile) variation in cervical length after tocolysis was stopped was 3 (0, 8) mm, and ranged from –13 to 26 mm. The median (first, third quartile) time interval from tocolysis to delivery was 53.0 (35.0, 70.0) days, with 45 (41.3%) patients delivered before 37 weeks. After adjustment for cervical length before admission and parity and gestational age on admission, the assessment of the variation in cervical length after successful tocolysis did not improve the predictive value of transvaginal sonography for the risk of preterm delivery (odds ratio 0.97; 95% confidence interval 0.90–1.03; P = .27).
CONCLUSION: To repeat ultrasonographic cervical length measurement after successful tocolysis for preterm labor is useless.
Different strategies have been developed to refine the risk of preterm delivery, particularly in cases with suspected preterm labor. One possibility is to rely on transvaginal sonography to measure and examine the length and shape of the cervix. Indeed, transvaginal ultrasound examination of the cervix improves the diagnosis of preterm labor with intact membranes.5,6,9–12 This has proven to be a more accurate means of predicting preterm delivery than digital examination of the cervix.5–7,9
All publications regarding the predictive value of transvaginal sonography have measured the cervical length before or after initiation of tocolysis.5,6,9–12 This study aimed to 1) describe the ultrasonographic modifications of the cervix among undelivered women after tocolysis was stopped, and 2) assess whether cervical length after a successful tocolysis could improve the predictive value of the ultrasound examination on the risk of preterm delivery.
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MATERIALS AND METHODS |
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In our department, digital examinations of the cervix are not performed routinely in preterm labor. Indeed, since our publication regarding the predictive value of fetal fibronectin and transvaginal sonographic measurement of cervical length for the risk of preterm delivery,12 we use a cervical length of 26 mm or less by transvaginal sonography instead of digital examination when making decisions regarding the admission and treatment of patients with regular and painful uterine contractions (at least 2 per 10 minutes) before 34 weeks. In France, the Creasy criteria are not used to define preterm labor. Moreover, no rigorous clinical criteria are routinely used. Therefore, and considering the better predictive value of transvaginal sonography, we elected to use this method with a threshold of 26 mm or less as a main definition of preterm labor before admission.
Inclusion criteria comprised a single pregnancy, regular and painful uterine contractions (at least 2 per 10 minutes) at 24+0 to 33+6 weeks of gestation, and a cervical length of 26 mm or less at admission as measured in a sagittal plane by transvaginal ultrasonography.
The following standardized technique was used. A General Electric GE 700 ultrasound machine (GE Healthcare, Vélizy, France) equipped with a 7.5-MHz transvaginal transducer was used. Each examination was performed with an empty bladder to avoid a deceptively elongated image of the cervix. The transducer was positioned on the cervix and identified the cervical canal, and both the internal and the external os of the cervix. Pressure on the transducer was relaxed to the point at which the image blurs, and enough gentle pressure was then reapplied to recreate a clear cervical image. A free interval of approximately 1 minute was allowed to minimize the problem of excessive transducer pressure and artificial lengthening of the cervix. A total of 3–5 measurements were then made in the absence of contractions, and the shortest one was kept for analysis. The reported interobserver and intraobserver variability of ultrasound measurement of cervical length is approximately 10%.12,13
Exclusion criteria included age younger than 18 years, a history of cervical incompetence with cerclage, cervical dilatation of more than 3 cm, the presence of chorioamnionitis, abnormal nonstress test before tocolysis, placenta praevia, abruptio placenta, preterm rupture of the membranes, hydramnios, intrauterine growth restriction, preeclampsia or gestational hypertension, multiple pregnancy, or any other maternal and/or fetal pathology mandating preterm delivery. Informed consent was obtained from all patients and the local ethics committee approved the study.
After admission, intravenous tocolysis was given with nicardipine, started at 1 mg/h and increased up to 4 mg/h if necessary until sedation of uterine contractions. Betamethasone 12 mg was systemically given intramuscularly and repeated after 24 hours. When delivery was delayed by 48 hours, intravenous tocolysis was stopped, and no other medication was given.
Within the next 6 hours, cervical length was again measured by transvaginal ultrasonography. The result of this latter measurement was not disclosed to the obstetric team and, therefore, did not influence subsequent patient management. Women were then discharged and subsequently followed up as outpatients, unless there was recurrence of uterine contractions. Strict bed rest was not recommended.
Data regarding maternal age, parity, and gestational age at admission and at delivery, and cervical length measured by transvaginal ultrasonography on admission and after tocolysis was stopped were collected. The primary outcome was the risk of delivery before 37 weeks of gestation.
Predictive analyses were based on logistic regression models, with computation of odds ratios and 95% confidence intervals. Univariable models were first fitted, with the following predictors tested: parity (0, 
1), gestational age on admission (< 30 weeks, 30 weeks), cervical length on admission (continuous), and cervical length after tocolysis (continuous). Multivariable logistic model was used to identify the set of prognostic variables, on the basis of likelihood ratio test. Assumptions of linearity of the effect of cervical lengths on admission and after tocolysis on the logistic scale were tested using generalized additive models.14
All tests were 2-tailed. Statistical significance was defined by P values of .05 or less. All analyses were performed on Statistical Application System (SAS Institute, Cary, NC) and S-Plus 2000 (MathSoft, Seattle, WA) software packages.
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RESULTS |
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The general and obstetric characteristics of the study population are indicated in Table 1. Among these 109 patients, the median (first, third quartile) parity was 1.0 (0, 1) and 52 (47.7%) were nulliparous. Four of these patients had a history of preterm delivery. Forty-five patients (41.3%) delivered before 37 weeks.
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The median (first, third quartile) interval from tocolysis to delivery was 53.0 (35.0, 70.0) days. The median (first, third quartile) variation in cervical length after tocolysis was stopped was 3 (0, 8) mm and ranged from –13 to 26 mm. Thus, 25% of the patients in preterm labor showed an increase in cervical length of more than 8 mm after successful tocolysis, and 25% showed a decrease in cervical length despite successful tocolysis and the absence of uterine contractions.
Table 2 shows the predictive analyses for preterm delivery, using univariable logistic regression models. Three variables were selected as having a predictive value: parity (P = .033), cervical length on admission (P < .001), and cervical length after tocolysis (P = .001). By contrast, the change in cervical length after tocolysis failed to show any predictive value (P = .31).
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Parity, gestational age on admission, cervical length on admission, and change in cervical length after tocolysis were introduced simultaneously in a multivariable logistic regression model. Only cervical length on admission remained predictive of the risk of preterm delivery after adjustment for gestational age on admission and parity (P = .001) (Table 3).
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DISCUSSION |
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Moreover, in this previous study, we showed that fetal fibronectin and cervical length were approximately equivalent in their ability to distinguish between patients at high and low risk for preterm delivery. For physicians equipped to perform transvaginal ultrasonography, however, the additional information about the fibronectin level provides only slight benefits. Therefore, we did not use fetal fibronectin levels in the current study.
Iams et al15 reported that the clinical value of funneling as a predictor of preterm delivery was similar to that of cervical length, but the data regarding funneling were confounded by substantial variation among operators, despite quality-control measures. Therefore, in this study, we only used cervical length to assess the risk of preterm delivery.
After successful tocolysis, changes in cervical length among undelivered women appear to be frequent. However, the ultrasonographic study of these modifications of the cervix does not improve the predictive value of cervical length measured before initiation of tocolysis on the risk of preterm delivery.
An increase in cervical length of more than 8 mm after successful tocolysis was seen in 25% of our population. The mean cervical length on admission was 18 mm; therefore, an increase in cervical length of more than 8 mm is unlikely to be explained by the variability of the technique only.12,13 Cervical lengthening might be explained by a reconstruction of the cervix from the lower uterine segment obtained after relaxation of the myometrium under tocolytics, assuming that the cervix and the lower uterine segment could behave as one functional unity rather than two different anatomical entities. However, cervical lengthening was not more predictive of the outcome, probably because it is only reflecting a temporary relaxation of the myometrium under tocolytics that present a short duration of efficiency.
Meis et al16 have recently reported that weekly injections of 17 alpha-hydroxyprogesterone caproate resulted in a substantial reduction in the rate of recurrent preterm delivery among women with a history of 1 or more preterm delivery. It would be interesting to investigate the predictive value of the cervical length measured before and after initiation of treatment with progesterone to predict which patients would be more likely to respond effectively to this treatment.
It is also interesting to note that a decrease in cervical length, observed among 25% of our patients despite successful tocolysis, did not alter the predictive value of cervical length measured before initiation of tocolysis on the risk of preterm delivery. Knowledge of the shortening of the cervix despite of tocolysis probably adds little information to that of a cervical length of 26 mm or less before treatment.
Because this was a cohort study, planned to be conducted for 20 months on the basis of consecutive inclusions of eligible patients, no computation of sample size was performed. Nevertheless, we computed the statistical power to detect several odds ratios for a 1–standard deviation increase in the cervical length,17 considering a 40% prevalence of preterm delivery, and a sample size of 109. In the absence of exposure measurement error, the statistical power was above 90% to detect an odds ratio of 2 for a 1–standard deviation increase in cervical length.
Ultrasound assessment of cervical changes after successful tocolysis did not improve the predictive value of cervical length measured by transvaginal sonography before initiation of tocolysis on the risk of preterm delivery. Therefore, to repeat transvaginal sonography in this population is useless.
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Footnotes |
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doi:10.1097/01.AOG.0000143254.27255.e9
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REFERENCES |
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2. Blondel B, Du Mauzaubrun C, Bréart G. Enquête nationale périnatale 1995. Rapport de fin d'étude. Edition INSERM. Paris: Février; 1996.
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5. Gomez R, Galasso M, Romero R, Mazor M, Sorokin Y, Goncalves L, Treadwell M. Ultrasonographic examination of the uterine cervix is better than cervical digital examination as a predictor of the likelihood of premature delivery in patients with preterm labor and intact membranes. Am J Obstet Gynecol 1994;171:956–64.[Medline]
6. Iams JD, Paraskos J, Landon MB, Teteris JN, Johnson FF. Cervical sonography in preterm labor. Obstet Gynecol 1994;84:40–6.
7. Berghella V, Tolosa JE, Kuhlman K, Weiner S, Bolognese RJ, Wapner RJ. Cervical ultrasonography compared with manual examination as a predictor of preterm delivery. Am J Obstet Gynecol 1997;177:723–30.[Medline]
8. Iams JD, Newman RB, Thom EA, Goldenberg RL, Mueller-Heubach E, Moawad A, et al. The National Institute of Child Health and Human Development Network of Maternal-Fetal Medicine Units. Frequency of uterine contractions and the risk of spontaneous preterm delivery. N Engl J Med 2002;346:250–5.
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12. Rozenberg P, Goffinet F, Malagrida L, Giudicelli Y, Perdu M, Houssin I, et al. Evaluating the risk of preterm delivery: a comparison of fetal fibronectin and transvaginal sonographic measurement of cervical length. Am J Obstet Gynecol 1997;176:196–9.[Medline]
13. Iams JD, Johnson FF, Sonek J, Sachs L, Gebauer C, Samuels P. Cervical competence as a continuum: a study of ultrasonographic cervical length and obstetric performance. Am J Obstet Gynecol 1995;1721:1097–103.
14. Hastie TJ, Tibshirani RJ. Generalized additive models. London: Chapman and Hall; 1990.
15. Iams JD, Goldenberg RL, Meis PJ, Mercer BM, Moawad A, Das A, et al. The length of the cervix and the risk of spontaneous premature delivery. National Institute of Child Health and Human Development Maternal Fetal Medicine Unit Network. N Engl J Med 1996;334:567–72.
16. Meis PJ, Klebanoff M, Thom E, Dombrowski MP, Sibai B, Moawad AH, et al. Prevention of recurrent preterm delivery by 17 alpha-hydroxyprogesterone caproate [published erratum appears in N Engl J Med. 2003;349:1299]. N Engl J Med 2003;348:2379–85.
17. Tosteson TD, Buzas JS, Demidenko E, Karagas M. Power and sample size calculations for generalized regression models with covariate measurement error. Stat Med 2003;22:1069–82.[Medline]
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