Summary of main findings
The primary purpose of the current meta-analysis was to estimate both the inter- and intra-methods reliability of using ultrasound compared to DVE in detecting cervical dilation measures. In this systematic review, ten studies were included. Overall, the random model showed poor reliability between the two methods. This can be because the women participating in these studies were not homogenous in terms of parity.
The subgroup analysis showed that the correlation between DVE and ultrasound in nulliparous women was poor, while this correlation in multiparous women was moderate. This means that in multigravida women, ultrasound measurements and digital examinations for cervical measures during labor are consistent. Overall, the pooled data indicated a low value of ultrasound diagnosis, resulting in conflicts with independent studies.
DVE is still the most commonly utilized method to assess cervical dilatation, fetal presentation, fetal position, and fetal descent during all stages of labor. However, DVE is associated with pain and the risk of infection. Therefore, clinicians tried to replace DVE with other methods, such as trans-perineal ultrasound.
The texture of the cervix changes dramatically after the first birth. Some women undergo rupture of the cervix at birth. Therefore, it is logical that multipara women would have a differently shaped cervix compared with their nulliparous counterparts . It is also possible that the cervix drastically remodels, reorganizes, and softens during gestation. Thus, the consistency and integrity of the cervix vary at different gestational age. As the fetus descends to the pelvis, more pressure is placed on the cervix. Hence, the length of the cervix is expected to shorten as a pregnancy progresses .
This is especially true in nulliparous women as the fetal descent happens during the last 4 weeks of pregnancy, and it is a slow descent, rather than a fast one, as seen in multiparous pregnancies. Moreover, women with elongated cervix might have more fiber in the cervix, making the cervix’s mechanics and structure different from those with the shorter cervixes .
In this study, we found a low value of ultrasound diagnosis, which conflicts with the independent studies. This could be due to the high heterogeneity found in our pooled data. The resolution lies in more sample size, which translates to conduct more quality RCTs. Furthermore, some of the included studies had very low sample sizes, and the effect of confounders such as the timing of membrane rupture, was not apparent. The studies also failed to mention whether the data were collected during the active or latent phase of labor.
The preliminary results of Zimerman et al. showed that ultrasound to detect cervical dilation is considered problematic . However, Hassan et al. showed that the correlation coefficient between ultrasound measurements and DVE is relatively high (r = 0.82, P = 0.05) [11, 12].
Also, Wiafe et al. in a systematic review showed a high correlation between ultrasound and digital examination of the cervix for detecting cervical dilation. Still, there was no significant difference in terms of success rate . The discrepancy between the present study and the Wiafe et al.’s study may be related to the fact that they recruited five studies. The heterogeneity in their meta-analysis was high (I2 = 96%), and they did not follow the DTA method.
DVE is the accepted clinical procedure for the detection of cervical dilatation during labor . However, DVE is a manual procedure that heavily depends on the providers’ experience. It is therefore, considered an imprecise measurement if conducted by inexperienced clinicians . In addition, examination and manipulation of the cervix might cause discomfort to women. In contrast, in ultra-sonographic cervical dilatation measurement, the uterine cervix is left intact, and natural contour is preserved . Also, cervical dilation changes in labor according to studies that used cervical ultrasound markers (clips) over time. Thus, two examiners may differ and yet both might be accurate . Martorelli et al. also concluded that transvaginal ultrasound before the onset of labor in women with gestational age > 40 weeks might help predict failed labor induction. Still, it should not be used for performing a cesarean section .
Strengths and limitations
This was the first systematic review to compare the reliability of ultrasound (TPUS or TLUS versus digital examination in detecting cervical dilation. The quality of the included studies was good, and most studies were free of serious biases.
Several limitations existed in this meta-analysis: (1) three studies failed to report parity; hence we were unable to include these studies in our subgroup analysis; (2) some other confounders such as the timing of rupture of member and the active or passive phases of labor were not evident; and (3) the sample size of the included studies was very small. These limitations could have contributed to heterogeneity substantially.
According to this systematic review, the digital examination can be replaced by trans-perineal ultrasound in multiparous women, while using this method in nulliparous women needs more thorough studies.