# The Structure and Function of the Cervix During Pregnancy Nott, J.P., Bonney, E.A., Pickering, J.D., Simpson, N., 2016. The structure and function of the cervix during pregnancy. Translational Research in Anatomy. [LINK](https://doi.org/10.1016/j.tria.2016.02.001) >[!Abstract] > The structure of the cervix is integral to the maintenance of pregnancy, keeping the developing baby in utero and forming a barrier to the ascent of microorganisms from the vagina. Weakness of the cervix may lead to deficiency of this barrier and is associated with subsequent preterm birth. The underlying cause of this structural weakness is poorly understood. In this paper we review the structure and function of the cervix before and during pregnancy. The causes of mechanical failure of the cervix during pregnancy are described, with a specific focus on the internal cervical os. We highlight the role of the internal cervical os in causing preterm birth and discuss research techniques that may provide further insight into its function during pregnancy. It is hoped that clinical translation of this knowledge will enable the early and appropriate identification of women who will benefit from strategies to reinforce the internal os and so reduce the incidence of preterm birth. ## Notes - during pregnancy, the function of the cervix is to retain and protect the growing baby - the cervix plays an important role in conception, pregnancy, and childbirth - cervical mucus is produced by the cervix - the cervix is held in place by paired ligaments on either side - cervix anatomy - the cervix must undergo a dramatic process of remodelling to allow for passage of the foetus - the cervix changes throughout the menstrual cycle - cervical mucus is regulated by oestrogen and progesterone - cervical mucus during ovulation - cervical mucus during luteal phase - the cervical canal is at its widest during the Follicular Phase - during labour the cervix softens, shortens and dilates %% [Open in Zotero](zotero://select/items/@Nott_2016) [[(Nott et al, 2016) The structure and function of the cervix during pregnancy.pdf]] ### [[the cervix plays an important role in conception, pregnancy, and childbirth]] > The cervix is integral to conception, the maintenance of pregnancy, and timely delivery of the baby. Throughout gestation the cervix must remain closed despite multiple forces acting upon it [1]. This enables it to act as a barrier to the ascent of vaginal microorganisms and to retain the growing fetus within the uterus. ^y7fw4 ### [[during labour the cervix softens, shortens and dilates]] > At term and during labour, the cervix must soften, shorten and dilate to allow for the baby's passage. ^m1bzl ### [[cervix anatomy]] > The cervix is a firm, cylindrical structure situated at the lower pole of the uterine corpus. The length of a normal adult non-pregnant cervix is approximately 25 mm, with an anteroposterior diameter ranging between 20 and 25 mm and a transverse diameter of 25e30 mm, although considerable variations occur due to age, parity and stage of menstrual cycle [3e5]. The cervix is divided into two portions that lie above and below the vaginal reflection, the portio supravaginalis and portio vaginalis respectively. Outside of pregnancy the cervical canal is collapsed, firm, and fusiform in shape. It ensures communication between the cavity of the corpus and the lumen of the vagina, and is bounded by the internal and external o ^pchiz ### [[the cervix is held in place by paired ligaments on either side]] > The cervix is held in place by paired ligaments on either side: the uterosacral and cardinal (transverse cervical) ligaments. The uterosacral ligaments run from the posterior and lateral supravaginal portions of the cervix to the middle three sacral vertebrae [5]. Near the most distal attachment at the cervix, the uterosacral ligaments interlace with their respective cardinal transverse ligaments for approximately 2 cm, before running to their proximal attachments [6]. It is thought that the uterosacral ligaments help to maintain the uterus in its typically anteverted state. ^ozuiq ### [[cervical mucus is regulated by oestrogen and progesterone]] > The manufacture and secretion of cervical mucus by the cervical columnar cells throughout the menstrual cycle is regulated by oestrogen and progesterone [28]. ^g6vj5 ### [[cervical mucus during ovulation]] >During ovulation the mucus produced has a stretchy and stringy consistency (spinnbarkeit), presumably facilitating the migration of sperm by providing an environment that is optimal for ascent, storage and survival of sperm [28]. ^rx6ux ### [[cervical mucus during luteal phase]] >Following ovulation and conception, the secretion of progesterone by the corpus luteum predominates in the second half of the cycle. The mucus reduces in fluidity and volume, becoming tacky and viscous, presumably discouraging spermatozooal and microbial ascent into the uterus. ^yiqiz ### [[the cervix changes throughout the menstrual cycle]] > The cervix demonstrates a considerable amount of anatomic (and physiologic) variation throughout the menstrual cycle. This is required to either promote or prevent the passage of spermatozoa and also to facilitate endometrial shedding. ^gipnt ### [[the cervical canal is at its widest during the Follicular Phase ]] > It was observed that in the menstrual and proliferative phases the cervical canal was at its widest, and the cervical mucosa more serrated, when compared to the secretory phase (Fig. 1) [4]. This has been subsequently supported and further quantified in magnetic resonance imaging (MRI) studies in which the mean width of the cervical canal was 4.5 mm in the follicular phase and 3.8 mm in the secretory phase of the menstrual cycle [25]. This study also demonstrated that the overall width and length of the cervix were greatest in the follicular phase of the menstrual cycle [25]. ^qas-e ### the cervix is able to narrow and widen at the junction between the corpus and the cervix > In a separate radiographic study that sought to determine the site of the isthmus, it was observed that the internal os appeared tightly closed during the luteal phase of the ovarian cycle and appeared to relax before the onset of menstruation [26]. Using a technique termed ‘direct hysterography,’ the author observed that lipiodol injected into the uterine cavity was retained between one to three hours in the follicular phase, between four to eight hours in the luteal phase, and less than 30 min two days prior to menstruation [26]. These results indicate that the cervix is able to narrow and widen at the junction between the corpus and the cervix. ### [[cervical mucus is produced by the cervix]] > The main product of the human cervix is cervical mucus [27]. ^xyzbq ### [[during pregnancy, the function of the cervix is to retain and protect the growing baby]] > The function of the cervix from this moment is to retain and protect the growing conceptus. An effective barrier is primarily accomplished through retaining a sufficient length of closed cervix within which the mucus plug can deter ascent of microbes from the lower genital tract [29]. This is aided by maintaining sufficient strength at the level of the internal os to discourage descent of the fetal membranes and conceptus down the cervical canal, which may shorten this barrier and/or dislodge the mucus plug. ^ftxuj ### [[the cervix must undergo a dramatic process of remodelling to allow for passage of the foetus]] > As labour approaches the cervix must undergo a dramatic process of remodelling to allow for passage of the fetus (Fig. 3). Remodelling can be subdivided into four overlapping phases: softening, ripening, dilation and postpartum restoration [41]. The process as a whole is as a consequence of enzymatic processes that lead to alterations in collagen alignment and organization [40], resulting in both structural and mechanical change. ^ooelu