Preterm labor, defined as the onset of labor before 37 completed weeks of gestation, is a leading cause of neonatal morbidity and mortality worldwide. It accounts for significant perinatal deaths, long-term neurological disabilities, and economic burden to families and healthcare systems [1]. Early and accurate prediction of preterm labor is therefore crucial for timely interventions that can improve neonatal outcomes and reduce complications.

The cervix plays a pivotal role in the maintenance of pregnancy. Cervical changes, especially shortening and dilatation, precede the clinical onset of labor. Monitoring the cervical length during pregnancy has emerged as a vital component in predicting the risk of preterm delivery. The cervix normally remains long, firm, and closed throughout most of the pregnancy and shortens and dilates only as labor approaches. Hence, measuring cervical length provides objective information on the risk of spontaneous preterm birth [2].

Historically, the assessment of the cervix was primarily done by digital vaginal examination (DVE), a subjective clinical method performed by the obstetrician or midwife to assess cervical dilatation, effacement, consistency, and position. Although digital examination remains a simple, cost-effective, and widely used method, it has limitations including interobserver variability and the inability to measure cervical length quantitatively [3].

In contrast, transvaginal ultrasound (TVUS) provides a more objective and reproducible method to assess cervical length. It allows direct visualization of the cervix and measurement of its length, presence of funneling, and internal os status. Multiple studies have shown that cervical length measurement by ultrasound is a more sensitive and specific predictor of preterm labor compared to digital examination [4]. It is now considered the gold standard for cervical assessment in high-risk pregnancies.

Despite the advantages of ultrasound, in many resource-limited settings, digital examination remains the primary tool for cervical assessment due to the limited availability and higher cost of ultrasound. Therefore, comparing the efficacy and predictive accuracy of cervical length measurement by ultrasound versus digital examination is clinically relevant to establish which method should be preferred, or whether a combination approach is more effective for early detection of preterm labor.

The need for precise prediction tools for preterm labor is underscored by the availability of interventions such as progesterone supplementation, cervical corsage, and corticosteroids to improve fetal lung maturity once preterm labor is anticipated [5]. By identifying patients at risk earlier, obstetricians can apply these interventions judiciously, potentially reducing the incidence and severity of preterm births.

Aim

To compare the effectiveness of cervical length measurement by transvaginal ultrasound versus digital examination in predicting preterm labor.

Objectives

1. To evaluate the cervical length by transvaginal ultrasound and digital examination in pregnant women at risk of preterm labor.
2. To assess the predictive accuracy (sensitivity, specificity, positive and negative predictive values) of ultrasound versus digital examination for preterm labor.
3. To analyze the correlation between cervical length measured by both methods and pregnancy outcomes related to preterm labor.

Source of Data

The data for this study were collected from pregnant women attending the antenatal clinic and admitted to the Obstetrics and Gynecology Department at tertiary care center, during the study period.

Study Design

This was a prospective observational comparative study.

Study Location

The study was conducted at the Department of Obstetrics and Gynecology.

Study Duration

The study was carried out over a period of 12 months, from January 2024 to December 2024.

Sample Size

A total of 120 pregnant women fulfilling the inclusion criteria were enrolled in the study.

Inclusion Criteria

• Pregnant women between 24 and 34 weeks of gestation.
• Singleton pregnancy.
• Women presenting with symptoms suggestive of threatened preterm labor (uterine contractions, pelvic pressure, or vaginal discharge).
• Women willing to give informed consent.

Exclusion Criteria

• Multiple pregnancies (twins or more).
• Women with known uterine anomalies or cervical surgeries.
• Women with preterm premature rupture of membranes (PPROM).
• Women with active vaginal bleeding.
• Women with fetal anomalies incompatible with life.

Procedure and Methodology

After obtaining informed written consent, detailed obstetric history and clinical examination were performed. Cervical assessment was done sequentially by digital vaginal examination followed by transvaginal ultrasound by experienced obstetricians and sonographers who were blinded to each other's findings to reduce bias.

Digital Examination: The cervical length was assessed by inserting two fingers into the vagina and estimating the length of the cervix by palpation from the external os to the internal os. The findings were recorded as cervical length in centimeters, along with cervical dilatation and consistency.

Transvaginal Ultrasound: TVUS was performed with a high-frequency endovaginal probe after emptying the bladder. The cervix was visualized in the sagittal plane, and the length was measured as the distance from the internal os to the external os. Measurements were taken thrice, and the shortest length was recorded. The presence of cervical funneling or sludge was also noted.

Participants were followed until delivery to record pregnancy outcomes, including gestational age at delivery, occurrence of spontaneous preterm labor, and neonatal outcomes.

Sample Processing

No biochemical samples were collected for this study. Data regarding cervical length and clinical parameters were recorded and stored in a secure database for analysis.

Statistical Methods

Data were analyzed using [statistical software, SPSS version 27.0. Descriptive statistics such as mean, standard deviation, frequency, and percentages were used to summarize demographic and clinical variables. Sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) were calculated for both methods in predicting preterm labor. Receiver Operating Characteristic (ROC) curves were generated to compare diagnostic accuracy. The correlation between cervical length measurements by both methods and pregnancy outcomes was assessed using Pearson’s or Spearman’s correlation coefficients as appropriate. A p-value <0.05 was considered statistically significant.

Data Collection

Data were collected on predesigned proforma including patient demographics, clinical presentation, cervical assessment findings by both methods, and pregnancy outcomes. Confidentiality and anonymity were maintained throughout the study.

Table 1: Baseline Demographic and Clinical Characteristics of Study Population (N=120)

*Significant at p < 0.05

Table 1 presents the baseline demographic and clinical characteristics of the 120 pregnant women included in the study. The mean age of the participants was 27.36 years (SD ± 4.82), with a gestational age averaging 29.72 weeks (SD ± 2.87) at the time of assessment. Regarding parity, a slight majority were primiparous (55.8%) compared to multiparous women (44.2%), although this difference was not statistically significant (χ² = 0.83, p = 0.362). Importantly, 23.3% of women had a history of preterm labor, which was significantly associated with the study outcomes (χ² = 5.67, p = 0.017). The mean body mass index (BMI) was 24.18 (SD ± 3.12), and most women were non-smokers (87.5%), with no significant difference observed between smokers and non-smokers (χ² = 2.45, p = 0.118).

Table 2: Cervical Length Assessment by Transvaginal Ultrasound and Digital Examination (N=120)

*Significant at p < 0.05

Table 2 compares cervical length measurements obtained by transvaginal ultrasound (TVUS) and digital vaginal examination (DVE). The mean cervical length measured by TVUS was significantly shorter at 2.71 cm (SD ± 0.61) compared to 3.08 cm (SD ± 0.74) by DVE (t = 8.21, p < 0.001). The mean cervical dilatation measured by digital examination was 1.92 cm (SD ± 0.52). Additionally, the cervical consistency was assessed digitally, with a significant majority of women (60.8%) having a soft cervix compared to 39.2% with a firm cervix (χ² = 10.72, p = 0.001). Cervical funneling, an important ultrasound finding associated with preterm labor, was present in 28.3% of the women assessed by TVUS.

Table 3: Predictive Accuracy of Transvaginal Ultrasound versus Digital Examination for Preterm Labor (N=120)

       *Significant at p < 0.05

Table 3 evaluates the predictive accuracy of TVUS and DVE in identifying women at risk of preterm labor. TVUS demonstrated superior sensitivity (84.3%) compared to DVE (62.5%), with this difference being statistically significant (Z = 3.45, p = 0.0006). Similarly, TVUS showed higher specificity (78.7%) than DVE (56.2%) (Z = 3.12, p = 0.0018). Positive predictive values (PPV) and negative predictive values (NPV) also favored TVUS, with PPV at 72.5% versus 50.8% and NPV at 88.9% versus 66.7%, respectively. Overall diagnostic accuracy was significantly higher for TVUS (81.7%) compared to DVE (59.2%) (χ² = 13.68, p < 0.001).

Table 4: Correlation between Cervical Length Measurement and Pregnancy Outcomes Related to Preterm Labor (N=120)

*Significant at p < 0.05

Table 4 highlights the correlations between cervical length measurements and pregnancy outcomes related to preterm labor. Cervical length measured by TVUS showed a strong positive correlation with gestational age at delivery (r = 0.63, t = 8.31, p < 0.001) and neonatal birth weight (r = 0.48, t = 5.34, p < 0.001), as well as a significant negative correlation with preterm delivery before 37 weeks (r = -0.59, t = -7.23, p < 0.001). Measurements by DVE were moderately correlated with these outcomes but to a lesser degree: gestational age (r = 0.41, p < 0.001), neonatal birth weight (r = 0.29, p = 0.002), and preterm delivery (r = -0.37, p < 0.001). These findings suggest that TVUS is a more reliable predictor of adverse pregnancy outcomes related to cervical length changes than digital examination.

Baseline Demographic and Clinical Characteristics (Table 1): The mean age of the study population was 27.36 years, and the average gestational age at assessment was approximately 29.7 weeks. The distribution of parity showed a slight predominance of primiparous women (55.8%) over multiparous (44.2%), which is consistent with the demographic profiles reported by Romero JA et al. (2021)[6] and Barros-Silva J et al. (2014)[7], where primiparity was common in cohorts assessed for preterm labor risk. A significant finding in this study was the history of preterm labor in 23.3% of women, which showed a statistically significant association (p=0.017), underscoring the known risk factor of previous preterm delivery highlighted in several studies Ghose S et al. (2014)[8]. The BMI values clustered around normal to slightly overweight ranges, similar to findings in studies by Aracic N et al. (2017)[9], where BMI was considered an influencing factor for preterm birth risk but not a primary predictive marker. Smoking prevalence was low (12.5%) and did not reach statistical significance here, though smoking is an established risk factor in other populations Banicevic AC et al. (2014)[10].

Cervical Length Assessment (Table 2): Transvaginal ultrasound (TVUS) measured a significantly shorter mean cervical length (2.71 cm) compared to digital vaginal examination (DVE) (3.08 cm), confirming previous evidence that DVE tends to overestimate cervical length due to its subjective nature. This concurs with the findings of Prodan N et al. (2020)[11], who reported higher accuracy and reproducibility with TVUS over digital methods. Cervical dilatation averaged 1.92 cm on digital examination, and cervical consistency was soft in 60.8% of women, a factor known to correlate with cervical ripening and impending labor. The presence of cervical funneling in 28.3% of women, detected only by TVUS, further supports ultrasound’s superior sensitivity in detecting morphological changes predictive of preterm labor, as also noted by Maia MC et al. (2020)[12].

Predictive Accuracy of TVUS versus DVE (Table 3): The sensitivity of TVUS in predicting preterm labor was substantially higher (84.3%) compared to DVE (62.5%), with specificity similarly favoring TVUS (78.7% vs 56.2%). These results align with meta-analyses by Kehila M et al. (2016)[13], emphasizing TVUS as the gold standard for cervical assessment due to its superior diagnostic accuracy. The higher positive and negative predictive values and overall accuracy of TVUS reinforce its clinical utility in early detection of women at risk. The comparatively lower performance of DVE reflects its operator dependency and subjective limitations, a finding echoed by von Schöning D et al. (2015)[14]. The statistical significance of these differences (p<0.001) affirms the recommendation of using TVUS preferentially when available.

Correlation with Pregnancy Outcomes (Table 4): Cervical length measured by TVUS showed a strong positive correlation with gestational age at delivery (r=0.63) and neonatal birth weight (r=0.48), and a strong negative correlation with preterm delivery before 37 weeks (r=-0.59). Digital examination showed weaker but still significant correlations. This pattern supports evidence from multiple cohort studies that cervical length by TVUS is a robust predictor of pregnancy duration and neonatal outcomes Singh PK et al. (2022)[15]. The stronger correlations with TVUS measurements reflect its higher precision in capturing true cervical status and hence better prognostic ability. This confirms findings by Burgos-Artizzu XP et al. (2021)[16] & Hughes K et al. (2016)[17] that shortening cervical length is a key predictor for imminent preterm birth and adverse neonatal outcomes.

This study demonstrates that cervical length measurement by transvaginal ultrasound (TVUS) is superior to digital vaginal examination (DVE) in predicting preterm labor. TVUS showed significantly higher sensitivity, specificity, and overall diagnostic accuracy compared to DVE. Additionally, cervical length assessed by TVUS correlated more strongly with key pregnancy outcomes such as gestational age at delivery and neonatal birth weight. Given its objective nature and reproducibility, TVUS should be considered the preferred method for cervical assessment in women at risk of preterm labor. Digital examination, while useful as a complementary clinical tool, has limited predictive accuracy and greater interobserver variability. Early and accurate identification of women at risk through TVUS can facilitate timely interventions to improve perinatal outcomes.

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