The mechanical role of the cervix in pregnancy
Introduction
The cervix is contiguous with the lower part of the uterus. Its proximal portion is located in the abdomen and its distal portion in the vagina. It has a narrow central canal which runs along its entire length, connecting the uterine cavity and the lumen of the vagina. The opening of this canal into the uterus is called the internal os and the opening into the vagina the external os (Fig. 1A). During pregnancy, the primary biomechanical function of the cervix is to maintain the fetus within the uterus. This requires withstanding multiple forces from the uterus, including the weight of the growing fetus and amniotic sac, as well as passive pressure from the uterine wall. Then, in a dramatic reversal of roles, the cervix markedly softens, shortens and dilates to allow delivery of the fetus. Shortly after delivery, the cervix reforms into its previous shape and consistency. How the cervix manages these complex dynamic changes is an interesting and understudied biomechanics problem.
Critical problems can occur when the timing and extent of the biomechanical changes are altered. Specifically, premature softening, shortening and dilation, which may be considered early mechanical failure, occurs in cases of spontaneous preterm birth (sPTB). The underlying pathophysiology of these changes is poorly understood despite that preterm birth affects 15 million babies annually, is the leading cause of childhood () death, and in 2013 was responsible for 1 million deaths (World Health Organization, 2014). The rate of preterm birth has significantly decreased by 2 decades of intense research effort into its pathophysiologies and associated molecular mechanisms. We believe that this lack of progress is partly due to lack of crosstalk between clinicians, engineers and basic scientists, and that progress will require multidisciplinary collaboration between previously distinct areas of expertise such as clinical obstetrics and engineering.
To begin that dialogue, here we provide an engineering framework for studying the mechanical function of the cervix during pregnancy. The main steps include modeling the material behavior of the cervix, characterizing the pelvic anatomy, capturing the appropriate contact conditions between the pelvic soft tissues, and understanding the relevant loading and boundary conditions. Accomplishing these tasks are a challenge because, in addition to understanding the basic material and anatomical parameters, one must consider the changes that occur throughout pregnancy to accommodate the growth and ultimate delivery of the fetus. However, overcoming these engineering challenges could lead to a better understanding of normal and pathological cervical deformation.
Section snippets
Scope of the problem
Preterm birth is defined as delivery between 20 weeks and 36 weeks+6 days gestation. Medically indicated preterm birth may result from maternal factors (e.g. preeclampsia or placenta previa) or fetal factors (e.g. oligohydramnios or growth restriction). Spontaneous preterm birth (sPTB) was formerly divided into two general categories, namely cervical dysfunction (cervical insufficiency or cervical incompetence) or preterm labor (typically thought to be the result of intrauterine infection or
The multi-scale mechanical environment of pregnancy
Studying and characterizing reproductive organs in real-time throughout gestation is understandably challenging. Pregnancy is a protected environment and accessing organs to measure either geometry or material properties during this time is difficult. There is a wide range of biologic length scales that determine the structural response of the reproductive organs during pregnancy (Fig. 1A). These factors include features of the pelvic anatomy and the hierarchal material characteristics of the
Cervical tissue mechanical properties
Characterizing the material behavior of human cervical tissue using either in vivo or ex vivo methodologies remains a challenge. Each methodology has advantages and drawbacks in terms of its ability to capture tissue remodeling characteristics of pregnant tissue or to obtain enough data to derive fully predictive 3D constitutive material equations. Animal models of pregnancy offer opportunities to test gestation-timed, genetically-altered, or chemically-treated samples. However, it is unclear
In summary: integrating strategies
Complementary methods to characterize cervical remodeling during pregnancy can help derive a fully descriptive material constitutive model for tissue behavior under mechanical loading. Mechanical characterization methodologies described here have limitations and advantages. In terms of capturing overall bulk tissue properties during cervical remodeling, in vivo QUS methods and mechanical aspiration offer noninvasive techniques to characterize cervical properties during pregnancy. These tools
Conflict of interest statement
None declared.
Acknowledgments
K.M., J.V., and R.W. acknowledge the support of the National Science Foundation BRIGE1125670 award, the Office of the Provost at Columbia University, and the Columbia University Medical Center Irving Institute for Clinical and Translational Research, which is supported by the National Center for Advancing Translational Sciences, National Institutes of Health through Grant no. UL1 TR000040. M.B. and E.M. gratefully acknowledge financial support by Swiss National Science Foundation, Grant no.
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