Sonography of the cervix at term gestation

Abstract

Abstract During pregnancy the firm cartilage-like consistency of the cervix is transformed to soft tissue. This transformation is called "cervical ripening" and enables the cervix to dilate and facilitate parturition. In 1960, the Bishop Score was introduced to assess cervical ripeness before induction of labour, which is determined by digital examination. The Bishop Score consists of five different aspects of the cervix, namely dilatation, effacement, consistency, position and station of the presenting part of the fetus. Since then, also assessment of preterm cervical ripeness and cervical incompetence is determined by the Bishop Score. Only recently, in the nineteen eighties, the usefulness of transvaginal sonography (TVS) of the cervix for the detection of cervical incompetence became apparent. The advantages of TVS compared to digital examination of the cervix were recognized shortly after its introduction. Early cervical ripening, resulting in changes at the internal cervical os, can be observed by TVS, even in the absence of dilatation. Nowadays, it is considered to be reproducible and easy to learn, even for inexperienced investigators. TVS of the cervix is now widely used as a screening method for preterm delivery in symptomatic and asymptomatic women. In the latest two decades, the clinical use of TVS of the cervix has further been expanded. There was need for a way to predict the outcome of labour induction at term more accurately than by the Bishop Score. In several studies pre-induction assessment of cervical ripeness by TVS has been compared with digital examination, but so far results are conflicting. It is striking that with the rapid evolvement of TVS in general clinical practice, only a few small studies report on physiological changes of the cervix preceding term labour. Knowledge of the physiology of the cervix at term is necessary to interpret TVS results in case of threatened preterm labour or before induction of labour at term. The general objective of this thesis was to study the cervix by TVS at term to obtain more insight in physiological changes preceding parturition, and to relate these changes or otherwise to the onset and course of spontaneous labour and to the need for and outcome of induced labour. Unfortunately, we found large variation in CL changes preceding spontaneous onset of labour, which restricts clinical utilization. However, a few recommendations can be made. CL has wide normal ranges throughout pregnancy. Before term, a short cervix is exceptional, however the chance of delivering in case of a short cervix is comparable to the flip of a coin. In contrast, preterm delivery is highly unlikely in case of a long cervix. At term, it is exactly the opposite. A long cervix at term is exceptional, although it does not at all exclude spontaneous onset of labour within short time. A short cervix at 37 weeks is reassuring, however spontaneous onset of labour may not be due until weeks. A short cervix on TVS at prolonged pregnancy may be in favour of expectant management. On the other hand, a short cervix at pre-induction TVS increases the chance of a vaginal delivery. Adding a maternal postural change to the examination improves the assessment of the functional status of the cervix. With knowledge of its limitations, TVS of the cervix can be used at term gestation.