Obstetrics & Gynecology Email Alerts
HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
QUICK SEARCH:   [advanced]


     

Obstetrics & Gynecology 2000;96:972-978
© 2000 by The American College of Obstetricians and Gynecologists
This Article
Right arrow Abstract Freely available
Right arrow Full Text (PDF)
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Similar articles in this journal
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by HIBBARD, J. U.
Right arrow Articles by MOAWAD, A. H.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by HIBBARD, J. U.
Right arrow Articles by MOAWAD, A. H.

ORIGINAL RESEARCH

Cervical Length at 16–22 Weeks’ Gestation and Risk for Preterm Delivery

JUDITH U. HIBBARD, MD, MAUREEN TART, MD and ATEF H. MOAWAD, MD

From the Division of Maternal-Fetal Medicine, Department of Obstetrics and Gynecology, University of Chicago, Pritzker School of Medicine, Chicago, Illinois.

Address reprint requests to: Judith U. Hibbard, MD, University of Chicago, Department of Obstetrics and Gynecology, 5841 South Maryland Avenue, MC 2050, Chicago, IL 60637, E-mail: jhibbard{at}babies.bsd.uchicago.edu


   Abstract
Top
Abstract
Materials and Methods
 Results
 Discussion
 References
 
Objective: To determine whether cervical length, as measured by transvaginal sonogram in asymptomatic gravidas at 16–22 weeks, is associated with risk of spontaneous preterm delivery.

Methods: In a prospective observational study in an unselected urban tertiary care population, cervical length was measured by transvaginal ultrasound during routine anatomic surveys in 760 gravidas at 16 0/7 to 22 6/7 weeks. The predictor variable was cervical length and the outcome variable was gestational age at delivery. Care providers were not masked to the results. Spontaneous preterm delivery was analyzed as before 37, 35, and 32 weeks. Shortened cervical length was defined by the tenth, fifth, and two and a half percentiles for our population. Yates-corrected {chi}2 was used to evaluate the significance on univariate analysis of the relative risk (RR) and 95% confidence intervals (CI). Multiple logistic regression analysis was used to control for background variables in evaluating the probability of pre-term delivery at less than 35 weeks. Sensitivity, specificity, positive and negative predictive values also were calculated.

Results: Cervical lengths were normally distributed (mean 38.5 ± 8.0 mm at 19.9 ± 1.5 weeks) independent of gestational age at measurement, and the tenth, fifth, and two and a half percentiles were 30, 27, and 22 mm, respectively. Eighty-five women delivered before 37 weeks, 51 before 35 weeks, and 27 before 32 weeks. Relative risks (95% CI) for spontaneous preterm delivery before 37 weeks were 3.8 (2.6, 5.6), 5.4 (3.3, 9.0), and 6.3 (3.0, 13.0) for the tenth, fifth, and two and a half percentiles, respectively; RRs for before 35 weeks were 4.5 (2.9, 6.9), 7.5 (4.5, 12.5), and 7.8 (3.6, 16.7); and for before 32 weeks were 5.2 (3.3, 8.3), 9.7 (5.8, 16.1), and 8.4 (3.6, 19.9), respectively. Multiple logistic regression analysis confirmed that cervical length was a significant predictor of preterm birth before 35 weeks, and that paras had a 43% greater risk compared with nulliparas. Sensitivity ranged from 13–44%, specificity 90–99%, positive predictive value 15–47%, and negative predictive value 80–98%.

Conclusion: Transvaginal measurement of cervical length during routine ultrasound at 16–22 weeks’ gestation in asymptomatic gravidas might help identify women at risk for spontaneous preterm delivery.

Cervical length measured by transvaginal sonogram, which is more accurate than transabdominal ultrasound1,2 or digital examination,3–5 has been inversely associated with spontaneous preterm delivery.5–8 Andersen et al6 described a risk of preterm delivery that increased to cervical length. That observation was confirmed in at-risk5,9 and general obstetric populations, unselected by risk factors.6–8,10,11 In an unselected low-risk obstetric population, Iams et al7 noted that the risk of spontaneous preterm delivery before 35 weeks increased more than sixfold and ninefold when measurements were less than 26 mm (tenth percentile) taken at 24 and 28 weeks, respectively. In an unselected Finnish population, Taipale and Hilesmaa8 found that shortened cervical length at 18–22 weeks’ gestation had a relative risk of spontaneous preterm delivery eight times that of women with a normal cervical length. However, that population was a homogeneous group with an impressively low incidence of spontaneous preterm delivery before 35 weeks (0.8%). As a result, sensitivity and positive predictive value of cervical length in that population were quite low. We investigated whether cervical length measured early in gestation (16–22 weeks) by transvaginal ultrasound would predict spontaneous preterm delivery in our unselected urban American tertiary center population.


   Materials and Methods
Top
Abstract
 Materials and Methods
Results
 Discussion
 References
 
We did this prospective observational study at the University of Chicago, Chicago Lying-In Hospital between July 1997 and July 1999. The population included all obstetric patients who had anatomic surveys in the ultrasound unit between 16 and 22 completed weeks’ gestation. Each woman also had to complete delivery during that time at the Chicago Lying-In Hospital. We incorporated transvaginal ultrasound measurements of cervical length into our comprehensive anatomic surveys of all obstetric patients scanned at those gestational ages. Such anatomic surveys were standard care at our institution. Patients were advised of the policy, and permission to do transvaginal sonograms was given verbally. The study was approved by the Human Investigations Committee as part of a protocol to select patients for an interventional trial. The approved interventional trial intended to randomly assign women with cervical lengths less than 26 mm to cerclage or bedrest. The trial was unsuccessful, with only eight women enrolled before it was terminated, and those women were not included in the current study. We continued to incorporate cervical length measurements in our anatomic surveys for this study. Measurements were stored in a computer database (OB Compass, Detroit, MI) and included in the report, so care providers were not masked to results. Subjects were not selected by risk factors. Because cervical length does not differ with parity,7,12 no distinction was made based on it. If more than one scan was done in that period, the first cervical length measurement was used for our study. Subjects were excluded if they did not meet gestational age requirements. Women with indications for preterm delivery from maternal or fetal conditions were excluded, as were those with cerclage placed in the current pregnancy, the aforementioned eight women enrolled in the interventional trial, and multiple gestations. Gestational age was assigned by accurate last menstrual period (LMP) or date of conception, or the earliest ultrasound measurement if it was more than 1 week discrepant in the first trimester, one and a half weeks in the second trimester, or the woman had an unknown LMP.

Sonographic measurements were done using an HDI 3000 instrument (Advanced Technology Laboratories, Seattle, WA) with a 4–8 MHz endovaginal probe according to a technique described by Iams et al.7,13 After consent, subjects were asked to empty their bladders. After the probe was inserted and a clear view of the endocervical canal was obtained, the probe was withdrawn to release the pressure until the image blurred. Slight pressure was then reapplied to obtain a clear image. The cervical length was measured along the closed endocervical canal between the triangular area of echodensity at the external os, and the V-shaped notch at the internal os. The shortest of three measurements was recorded. Three technicians trained in the technique by one of the authors (JUH) did all the scans and were supervised by attending faculty. Quality control was ensured in a manner similar to that of Iams et al.7,13 Each technician recorded cervical length measurements on ten consecutive women on videotape for review and critique. Intraobserver coefficient of variation of less than 10% was accepted.

Delivery information was collected from our computerized obstetric database, including gestational age, birth weight, Apgar scores, mode of delivery, nursery admission, and complications. The primary predictor variable was cervical length, and the primary outcome variable was gestational age at delivery. Funneling of the lower uterine segment was not chosen as a predictor because of the variable nature of the measurements.7 Spontaneous preterm delivery was defined and analyzed at the following three cutoff points: delivery before 37 weeks (the traditional definition for preterm delivery), before 35 weeks, and before 32 weeks, when most neonatal morbidity and mortality occurs and the time-focus of most neonatal intensive care units in this country. Indicated preterm deliveries were not included in analyses.

Cervical length distribution for our population was analyzed, and mean and standard deviations and percentiles were calculated. Normality was tested using descriptive statistics for skewedness, a histogram, and a normality plot (inverse normal versus cervical length), and a Kolmogorov-Smirnov test for normality. A Pearson correlation test was used to test for independence between cervical length and gestational age. Abnormal cervical length was defined as at or below the tenth, fifth, and two and a half percentiles. Those values were then used to examine our data for an association with preterm delivery, initially performing univariate analysis to estimate the risk of spontaneous preterm delivery before 37, 35, and 32 weeks. Yates corrected {chi}2 analysis was used to evaluate the significance of the relative risk (RR) and 95% confidence interval (CI). Multiple logistic regression analysis was done to estimate probability of spontaneous preterm delivery before 35 weeks while controlling for background variables. Variables considered in addition to cervical length were maternal age, race, marital status, payor status, parity, and substance abuse. The final model included maternal age, marital status, parity, and cervical length. Sensitivities, specificities, positive predictive values, and negative predictive values were also calculated. Two-tailed Student t test was used to compare groups, with P < .05 considered statistically significant. In our population, with a 7.5% incidence of spontaneous preterm delivery at less than 35 weeks, we assumed a 3% incidence of shortened cervix, an {alpha} of 0.05, and ß of 0.20, and calculated that 682 women would need to be studied to have an 80% power for shortened cervical length to detect a fourfold increase in risk for spontaneous preterm delivery before 35 weeks. A stratified analysis by weeks’ gestation at the time of cervical length measurement (16–17, 18–19, and 20–22 weeks) was done, but insufficient events in two of three groups precluded statistically meaningful results.


   Results
Top
Abstract
 Materials and Methods
 Results
Discussion
 References
 
During the study, 6399 women delivered at Chicago Lying-In Hospital. The overall preterm delivery rates and the spontaneous preterm delivery rates for that population are shown in Table 1Go. The original study population included 841 gravidas who had routine anatomic surveys, including transvaginal cervical length measurements, in our sonography unit, and also delivered at Chicago Lying-In Hospital during that period. Such a small percentage of the total deliveries was assessed at 16–22 weeks because a large percentage of women present for prenatal care later in gestation, and 20% arrive for delivery unregistered with no prenatal care at our institution. Fifty-five women were excluded from analysis because they had indicated preterm deliveries. The reasons for early delivery included preeclampsia, maternal medical complications, symptomatic placenta previa, intrauterine fetal death, severe fetal anomalies, and nonreassuring fetal well being. Maternal medical conditions leading to preterm delivery included one woman with severe systemic lupus erythematosis flare and another with life-threatening asthma exacerbation. Nonreassuring fetal indications for early delivery included discordant twins, abnormal fetal heart-rate tracings, and abnormal biophysical profiles. Twenty-six women were excluded because of cerclage placement or entry in the interventional protocol. The final study population consisted of 760 gravidas. Demographic information for the population is given in Table 2Go. The mean (± standard deviation [SD]) gestational age at cervical length measurement was 19.9 ± 1.5 weeks, and the mean ± SD gestational age at delivery for the entire population was 38.3 ± 2.8 weeks. Spontaneous preterm delivery before 37 weeks occurred in 11.2% of gravidas, before 35 weeks in 6.7%, and 3.6% of women delivered before 32 weeks (Table 1Go).


View this table:
[in this window]
[in a new window]
  		Table 1. Overall Preterm Delivery Rates and Spontaneous Preterm Delivery Rates for University of Chicago and the Study Population
 

View this table:
[in this window]
[in a new window]
  		Table 2. Demographic Characteristics
 
Cervical length measurements were normally distributed in our population, as shown in Figure 1Go, meeting criteria for normality by basic descriptive statistics, inspection of the histogram, and a normality plot. A Kolmogorov-Smirnov test also was done on the hypothesis that the cervical length data were normally distributed, and was accepted at a 1% significance level. The mean ± SD cervical length for the study population was 38.5 ± 8.0 mm. The tenth, fifth, and two and a half percentiles of cervical length were 30, 27, and 22 mm, respectively. The Pearson test for independence of cervical length measurement and gestational age at which the cervical length was measured verified independence between the two variables, P = .163. On univariate analysis the RRs, and 95% CIs for spontaneous preterm delivery at less than 37 weeks’ gestation were 3.8 (2.6, 5.6), 4.5 (2.9, 6.9), and 5.2 (3.3, 8.3), for the tenth, fifth, and two and a half percentiles of cervical length, respectively. For delivery before 35 weeks, RRs and 95% CIs were 5.4 (3.3, 9.0), 7.5 (4.5, 12.5), and 9.7 (5.8, 16.1), respectively, and for preterm delivery before 32 weeks were 6.3 (3.0, 13.0), 7.8 (3.6, 16.7), and 8.4 (3.6, 19.9), respectively. Results are shown in Figure 2Go. Multiple logistic regression analysis confirmed that cervical length was a significant predictor of preterm birth before 35 weeks’ gestation (P < .001). The analysis showed that for each 1-mm decrease in cervical length below the mean, the risk of preterm delivery increased by 10.3%. The model also showed that paras were at 43% greater risk for preterm delivery than nulliparas. The estimated probability of spontaneous preterm delivery before 35 weeks’ gestation determined by the multiple logistic regression model is shown in Figure 3Go. Table 3Go provides the predictive characteristics of shortened cervical length for the two and a half, fifth, and tenth percentiles for predicting spontaneous preterm delivery for each of the three defined gestational ages for preterm birth. Stratified multiple logistic regression analysis by gestational age increments at measurement of cervical length showed only 23 subjects at 16–17 weeks’ gestation, 144 at 18–19 weeks, and 376 between 20 and 22 weeks.



View larger version (26K):
[in this window]
[in a new window]
  		Figure 1. Normal distribution of cervical length measurements by transvaginal ultrasound at 16 to 22 weeks’ gestation.

 


View larger version (10K):
[in this window]
[in a new window]
  		Figure 2. Univariate analysis of relative risk ({circ}) and 95% confidence intervals ({blacktriangleup}) for spontaneous preterm delivery before 37, 35, and 32 weeks’ gestation according to transvaginal cervical length measurements at 16–22 weeks’ gestation for the tenth percentile (<= 30 mm), the fifth percentile (<= 27 mm), and the two and a half percentile (<= 22 mm).

 


View larger version (13K):
[in this window]
[in a new window]
  		Figure 3. Estimated probability of spontaneous preterm delivery before 35 weeks’ gestation based on multiple logistic regression analysis. The final model used the following equation: Log [{pi}/(1 - {pi}] = ß0 + ß1age + ß2marital + ß3para + ß4CL where {pi} = probability of preterm birth less than 35 weeks, and variables included in the model were maternal age, marital status, parity, and cervical length, respectively.

 

View this table:
[in this window]
[in a new window]
  		Table 3. Predictive Characteristics of Shortened Cervical Length and Spontaneous Preterm Delivery
 

   Discussion
Top
Abstract
 Materials and Methods
 Results
 Discussion
References
 
In this prospective observational study we found that unselected gravidas with short cervical lengths on endovaginal ultrasound at 16–22 weeks’ gestation were at significantly higher risk for early delivery. The group of women studied were from our general obstetric population of women enrolled for care at our urban tertiary center, where the spontaneous preterm delivery rate ranges from approximately 3% for delivery before 32 weeks’ gestation to 12% for delivery before 37 weeks. We analyzed the cervical length distribution of our population, using the tenth, fifth, and two and a half percentiles for our analysis because norms have not been established for that gestational age. We noted the risk of early delivery before 35 weeks was increased fivefold to tenfold over women with normal cervical lengths and the risk increased by 10.3% for every millimeter decrease in cervical length below the mean. Multiparas were at greatest risk. A stratified analysis by gestational age increments at which measurements were performed is desirable, but insufficient numbers, especially for 16–17 weeks’ gestation, made that unreliable. Caution should be used when applying those results to earlier gestational ages. Those observations, particularly for 18–22 weeks, might have important implications for patient selection and treatment to prevent preterm birth.

Cervical length measured by endovaginal ultrasound to predict preterm birth was first noted by Andersen et al,6 who found that in an unselected population of 113 gravidas, a cervix of less than 39 mm before 30 weeks’ gestation was a significant risk factor for early delivery and was inversely related to cervical length. That inverse relationship was verified by other investigators.2,7,8 Kushnir et al12 reported cervical length measurements in normal gravidas longitudinally, but did not study outcomes. Zorzoli et al14 also investigated cervical length longitudinally in 154 normal gravidas and did not find any association with gestational age at delivery. Tongsong et al10 studied 730 unselected women at 28 to 30 weeks’ gestation and found that cervical length less than 35 mm was associated with early delivery. Cervical length less than 26 mm, measured at 24 and 28 weeks’ gestation, was shown by Iams et al,7 in a large, multicenter, unselected heterogeneous American population, to have RRs for delivery before 35 weeks of 6.2 and 9.6, respectively. They also found a strong inverse relationship between cervical length and preterm delivery before 35 weeks’ gestation. Sensitivity and specificity of cervical length measurement at 24 weeks’ gestation were 37% and 92%, respectively. Only an 18% positive predictive value was achieved in that population with a 4.3% spontaneous preterm delivery rate before 35 weeks. Taipale and Hilesmaa8 investigated a large, homogeneous, unselected Finnish population at 18 to 22 weeks’ gestation, but the prevalence of preterm delivery before 35 weeks was only 0.8%. Although they found an eightfold increase in preterm delivery associated with a cervical length <=29 mm, the low prevalence rate resulted in only 19% sensitivity and 6% positive predictive value.

We also investigated an unselected obstetric population, and our study was similar to that of Iams et al7 in that our population was heterogeneous and was derived from a large urban American tertiary center population, but differed in that we studied women at earlier gestational ages. That age was used because it is when we do routine comprehensive anatomic surveys, and we wished to know whether the cervical measurements would be a useful addition to the examination, as well as being more convenient than a 24 or 28 week measurement. The cervical lengths for each percentile we measured were longer than those of Iams et al,7 possibly because of the earlier gestational age at which we did the scans, or perhaps they were inherent in our population. Cervical length at the tenth percentile was the same as that documented by Andersen et al.6 The sensitivity and specificity we achieved using the tenth percentile of cervical length, 41.2% and 90.7%, for delivery before 35 weeks, were almost identical to the 37.3% and 92.2% determined by Iams et al7 for their tenth percentile of cervical length. Our positive predictive value of 27%, better than the 17.8% of Iams et al,7 was because of our high spontaneous preterm delivery rate. Although the timeframe at which we measured cervical lengths was similar to that of Taipale and Hilesmaa,8 our results differed markedly in that our spontaneous preterm delivery rate before 35 weeks’ gestation was more than ninefold greater than that of the Finnish center. Our sensitivity (41.2%) and positive predictive value (27%) were much higher than those determined by Taipale and Hilesmaa,8 19% and 6%, respectively. We believe our population is much more comparable to urban tertiary centers in this country that also have high spontaneous preterm delivery rates. Multiparas were at greater risk of spontaneous preterm delivery than nulliparas, the opposite findings of Tai-pale and Hilesmaa,8 and an observation in need of further investigation.

One of the limitations of our study design was that measurements of cervical length were available to the clinicians ordering anatomic surveys, and we acknowledge that the study could have been improved by masking the results to all caregivers. Patients who had cerclage were excluded. The separate interventional trial was unsuccessful and was halted, and the few women enrolled in it were not included in this analysis. If a clinician suggested another form of intervention, such as bedrest, antibiotics, or tocolytics, with the assumption that it would decrease preterm delivery, and the intervention was actually effective, then our results would be biased toward a lower relative risk of delivering early than really exists and lower positive and negative predictive values. Additional bias might exist because women in this study were receiving prenatal care by 16–22 weeks’ gestation. Poor obstetric history, including previous preterm delivery, might have prompted them to register early, thus biasing our results toward greater relative risks than actually exist. Women who registered for care later or who presented for delivery with no prenatal care also had high preterm delivery rates. The study would be stronger if we had had information on previous conization procedures or previous preterm births, because parity was associated with risk of preterm birth in our population. Strengths of this investigation were that subjects were an unselected, representative population from our institution. Thus, we believe the results are applicable not only to our greater patient population but also to other urban centers in this country that serve populations with similar demographics.

Endovaginal cervical length measurement in routine anatomic surveys allowed us to more clearly see fetal intracranial anatomy and lumbosacral spine. We diagnosed placenta previa more accurately, and occasionally suspected a vasa previa or accreta. We did not investigate the cost-effectiveness of adding cervical length measurements to our anatomic survey, but we acknowledge that it might increase expenses of an obstetric sonography unit. Those expenses might include, with purchase of the endovaginal probe, the proper cleaning materials and procedures between patients.

This was an observational study, so we can make no recommendation on how to treat women with abnormal cervical lengths at routine anatomic survey. However, the method can be used to select patients for prospective interventional trials with the rationale that intervention before 24 weeks might be more effective than later intervention. Transvaginal cervical length measurement also can be used in combination with other markers for premature delivery, such as fetal fibronectin screening, serum alpha-fetoprotein, or serum alkaline phosphatase, in developing treatment strategies. Unfortunately, there is little to offer women in treatment to delay preterm delivery.


   Footnotes
 
PII S0029-7844(00)01074-7

Received April 27, 2000. Received in revised form July 6, 2000. Accepted August 31, 2000.


   References
Top
Abstract
 Materials and Methods
 Results
 Discussion
 References
 
1. Andersen HF. Transvaginal and transabdominal ultrasonography of the uterine cervix during pregnancy. J Clin Ultrasound 1991;19: 77–83.[Medline]

2. Okitsu O, Mimura T, Nakayama T, Aono T. Early prediction of preterm delivery by transvaginal ultrasonography. Ultrasound Obstet Gynaecol 1992;2:402–9.[Medline]

3. Sonek JD, Iams JD, Blumenfeld M, Johnson F, Landon M, Gabbe S. Measurement of cervical length in pregnancy: Comparison between vaginal ultrasonography and digital examination. Obstet Gynecol 1990;76:172–5.[Abstract/Free Full Text]

4. Jackson GM, Ludmir J, Bader TJ. The accuracy of digital examination and ultrasound in the evaluation of cervical length. Obstet Gynecol 1992;79:214–8.[Abstract/Free Full Text]

5. Berghella V, Tolosa JE, Kuhlman K. Cervical ultrasonography compared with manual examination as a predictor of preterm delivery. Am J Obstet Gynecol 1997;177:723–30.[Medline]

6. Andersen HF, Nugent CE, Wanty SD, Hayashi RH. Prediction of risk for preterm delivery by ultrasonographic measurement of cervical length. Am J Obstet Gynecol 1990;163:859–67.[Medline]

7. 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. N Engl J Med 1996;334:567–72.[Abstract/Free Full Text]

8. Taipale P, Hilesmaa V. Sonographic measurement of uterine cervix at 18–22 weeks’ gestation and the risk of preterm delivery. Obstet Gynecol 1998;92:902–7.[Abstract]

9. Berghella V, Daly SF, Tolosa JE, DiVito MM, Chalmers R, Garg N, et al. Prediction of preterm delivery with transvaginal ultrasonography of the cervix in patients with high-risk pregnancies: Does cerclage prevent prematurity? Am J Obstet Gynecol 1999;181:809–15.[Medline]

10. Tongsong T, Kamprapanth P, Srisomboon J. Transvaginal sonographic measurement of cervical length early in the third trimester as a predictor of preterm delivery. Obstet Gynecol 1995;86:184–7.[Abstract]

11. Heath VC, Southall TR, Souka AP, Elisseou A, Nicolaides KH. Cervical length at 23 weeks of gestation: Prediction of spontaneous preterm delivery. Ultrasound Obstet Gynecol 1998;12:312–7.[Medline]

12. Kushnir O, Vigil DA, Izquierdo L, Schiff M, Curet LB. Vaginal ultrasonographic assessment of cervical length changes during normal pregnancy. Am J Obstet Gynecol 1990;162:991–3.[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;172:1097–106.[Medline]

14. Zorzoli A, Soliani A, Perra M, Caravelli E, Galimberti A, Nicolini U. Cervical changes throughout pregnancy as assessed by transvaginal sonography. Obstet Gynecol 1994;84:960–4.[Abstract/Free Full Text]




This article has been cited by other articles:


Home page
 Obstet GynecolHome page
V. Berghella, A. O. Odibo, M. S. To, O. A. Rust, and S. M. Althuisius
Cerclage for Short Cervix on Ultrasonography: Meta-Analysis of Trials Using Individual Patient-Level Data
Obstet. Gynecol., July 1, 2005; 106(1): 181 - 189.
[Abstract] [Full Text] [PDF]

Home page
 Journal of Diagnostic Medical SonographyHome page
A. M. A. Madsen
Sonographic Methods of Evaluating Cervical Length: A Literature Review
Journal of Diagnostic Medical Sonography, May 1, 2003; 19(3): 157 - 165.
[Abstract] [PDF]

This Article
Right arrow Abstract Freely available
Right arrow Full Text (PDF)
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Similar articles in this journal
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by HIBBARD, J. U.
Right arrow Articles by MOAWAD, A. H.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by HIBBARD, J. U.
Right arrow Articles by MOAWAD, A. H.

HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS