Transvaginal Ultrasound Cervical Length as an Indicator of Successful Induction of labour

Abstract

Labor is the physiological process by which the fetus is expelled from the uterus to the outside world. It is defined as increase in myometrial activity or more precisely, a switch in the myometrial contractility pattern from "contractures" (long-lasting, low-frequency activity) to "contractions" (frequent, high intensity, high frequency activity), resulting in effacement and dilatation of the uterine cervix. labour and delivery are not passive processes by which uterine contractions push a rigid object through a fixed aperture. It is essential that during most of pregnancy the myometrium be able to stretch but remain quiescent. And, at the same time, the cervix must remain unyielding and reasonably rigid. Then, coincidentally with the initiation of parturition, the cervix must soften, yield, and become more readily dilatable. The fundus must be transformed from the relatively relaxed, unresponsive organ characteristic of most of pregnancy to one that will produce effective contractions that drive the fetus through the yielding (dilatable) cervix and on through the birth canal. Failure of this coordinated interaction between functions of the fundus and the cervix is indicative of an unfavorable pregnancy outcome. Induction of labour refers to the process whereby uterine contractions are initiated by medical or surgical means before the onset of spontaneous labour. Induction is indicated when the benefits to either the mother or the fetus outweigh those of continuing the pregnancy. Potential risks of induction include increased rate of operative vaginal delivery, caesarean birth, excessive uterine activity, abnormal fetal heart rate patterns, uterine rupture, maternal water intoxication, delivery of preterm infant due to incorrect estimation of dates, and possibly cord prolapse with artificial rupture of membranes. All of that mandate the finding of an appropriate and accurate way for prediction of the success rate of this process and justification of its results. The state of the cervix is one of the important predictors of successful labour induction. In 1964 developed a pelvic scoring system (Bishop Score) to predict inducibility by evaluating the position of the cervix as it relates to the vagina, the cervical consistency, dilation, effacement and station of the presenting part. The higher the score, the more favorable the cervix with a clinical trial showing a score of 6-7 or more associated with successful inductions. In modified Bishop's pre-induction cervical scoring system, effacement has been replaced by cervical length in cm, with scores as follows ( 0>3cm, 1=2-3cm, 2=1-2cm, 3<1cm). Bishop score is subjective and demonstrates intra- and interobserver variability. Transvaginal ultrasound has been proposed as a better predictor of the success of labour compared with the Bishop score. The advantage of the Bishop Score is that it evaluates cervical consistency and station, which may influence the outcome; it also requires less equipment. On the other hand, transvaginal ultrasound measurement of cervical length provides a more objective measurement. Many authors have tried to assess the Bishop score, transvaginal ultrasound, and both together aiming at evaluation of each of them as a method for prediction of labour induction concerning about their accuracy, tolerability, availability, etc. Some studies supposed that transvaginal ultrasonographic measurement of cervical length does not add any additional benefit to the prediction of cervical inducibility obtained by the Bishop score.