Chapter 28: SONOGRAPHIC EXAMINATION OF THE UTERINE CERVIX

INTRODUCTION

The uterine cervix plays a central role in the maintenance of normal pregnancy and in parturition. Thus, cervical disorders have been implicated in common obstetrical complications, such as "cervical insufficiency,"¹ preterm labor, and abnormal term parturition.² Yet, there is an incomplete understanding of the physiology and pathology of untimely cervical effacement and dilation during pregnancy. Mid-trimester cervical dilation is a major diagnostic and therapeutic challenge and a subject of intense debate among clinicians and researchers.

During most of normal pregnancy, the cervix remains firm and closed, despite a progressive increase in the size of the fetus and uterine distention. At the end of pregnancy and during labor, the cervix changes consistency (softens), shortens (effaces), and dilates to allow the expulsion of the conceptus. Labor, delivery, and the postpartum period are accompanied by dramatic changes in the uterine cervix.^{3,4,5,6,7,8,9, and 10} The term cervical ripening refers to the anatomic, biophysical, and biochemical processes that underlie the changes in cervical consistency, effacement, and dilation that generally precede the onset of spontaneous labor. Contrary to what was believed for many years, cervical ripening is an active metabolic process affecting the extracellular matrix components of the cervix. These changes increase cervical compliance. Untimely cervical ripening could result in complications of pregnancy. For example, failure of the cervix to ripen before myometrial activation at term (ie, onset of increased uterine contractility) may be the cause of a prolonged latent phase of labor; preterm premature cervical ripening may lead to mid-trimester spontaneous abortion or spontaneous preterm labor and delivery.

PHYSIOLOGY OF CERVICAL CHANGE IN PREGNANCY

The uterine cervix is essentially a connective tissue organ, with smooth muscle cells accounting for only 10% to 15% of the distal part of the cervix.¹¹ The ability of the cervix to retain the conceptus during pregnancy is unlikely to depend upon a traditional sphincteric mechanism. Indeed, perfusion of strips of human cervix with vasopressin, a hormone that stimulates smooth muscle contraction, results in a very modest contractile response in comparison to that induced by this hormone in strips from the uterine isthmus and the fundus, which contain more muscle.¹²

It is now understood that the normal function of the uterine cervix during pregnancy depends largely upon the regulation of connective tissue metabolism. This ...