How the cervix shortens: an anatomic study using 3-dimensional transperineal sonography and image registration in singletons and twins

Quantitative Ultrasound Analysis of Proximal and Distal Cervical Tissue Echogenicity in Premature Cervical Remodeling

OBJECTIVES

To determine whether a novel, noninvasive quantitative ultrasound (US) technique can detect differences in proximal and distal cervical tissue echogenicity in women with and without a shortened cervical length (CL).

METHODS

We conducted a retrospective case-control study of singleton pregnancies at 16 to 26 weeks' gestation in which a transvaginal US examination was performed to measure CL from 2013 to 2015. Initial CLs in cases and controls were less than 2.5 cm and 2.5 cm or greater, respectively. For each US image, a region of interest was selected in the proximal and distal cervical stroma, in both the anterior and posterior cervical lips. The Floyd-Steinberg dithering algorithm transformed grayscale pixels in each region of interest into a binary map. A histogram tabulated the number of black and white pixels, allowing determination of the percent echogenicity. The difference in the percent echogenicity was calculated by subtracting the distal cervical echogenicity (average of anterior and posterior lips) from the proximal cervical echogenicity (average of anterior and posterior lips).

RESULTS

Ultrasound images from 177 women were analyzed. There was a difference in the percent echogenicity (P < .0001) when comparing women with a short cervix (mean ± SD, 9.8 ± 10.1; n = 102) to women with a normal CL (17.2 ± 9.5; n = 75). Differences were attributable to changes in proximal (P < .008) rather than distal cervical echogenicity. Regardless of CL, the proximal cervix was more echogenic than the distal cervix.

CONCLUSIONS

A quantitative US analysis of cervical tissue can detect differences in echogenicity between the proximal and distal cervix in the second trimester. Proximal cervical echogenicity is lower with CL of less than 2.5 cm compared to a normal CL.

Biomechanics of the human uterus

The appropriate biomechanical function of the uterus is required for the execution of human reproduction. These functions range from aiding the transport of the embryo to the implantation site, to remodeling its tissue walls to host the placenta, to protecting the fetus during gestation, to contracting forcefully for a safe parturition and postpartum, to remodeling back to its nonpregnant condition to renew the cycle of menstruation. To serve these remarkably diverse functions, the uterus is optimally geared with evolving and contractile muscle and tissue layers that are cued by chemical, hormonal, electrical, and mechanical signals. The relationship between these highly active biological signaling mechanisms and uterine biomechanical function is not completely understood for normal reproductive processes and pathological conditions such as adenomyosis, endometriosis, infertility and preterm labor. Animal studies have illuminated the rich structural function of the uterus, particularly in pregnancy. In humans, medical imaging techniques in ultrasound and magnetic resonance have been combined with computational engineering techniques to characterize the uterus in vivo, and advanced experimental techniques have explored uterine function using ex vivo tissue samples. The collective evidence presented in this review gives an overall perspective on uterine biomechanics related to both its nonpregnant and pregnant function, highlighting open research topics in the field. Additionally, uterine disease and infertility are discussed in the context of tissue injury and repair processes and the role of computational modeling in uncovering etiologies of disease. WIREs Syst Biol Med 2017, 9:e1388. doi: 10.1002/wsbm.1388 For further resources related to this article, please visit the WIREs website.

The role of cervical length in women with threatened preterm labor: is it a valid predictor at any gestational age?

OBJECTIVE

To determine whether the predictive accuracy of sonographic cervical length (CL) for preterm delivery (PTD) in women with threatened preterm labor (PTL) is related to gestational age (GA) at presentation.

STUDY DESIGN

A retrospective cohort study of all women with singleton pregnancies who presented with PTL at less than 34 + 0 weeks and underwent sonographic measurement of CL in a tertiary medical center between 2007 and 2012. The predictive accuracy of CL for PTD was stratified by GA at presentation.

RESULTS

Overall, 1077 women who presented with PTL have had sonographic measurement of CL and met the study inclusion criteria. Of those, 223 (20.7%) presented at 24 + 0-26 + 6 weeks (group 1), 274 (25.4%) at 27 + 0-29 + 6 weeks (group 2), 283 (26.3%) at 30 + 0-31 + 6 weeks (group 3), and 297 (27.6%) at 32 + 0-33 + 6 weeks (group 4). The overall performance CL as a predictive test for PTD was similar in the 4 GA groups, as reflected by the similar degree of correlation between CL with the examination to delivery interval (r = 0.27, r = 0.26, r = 0.28, and r = 0.29, respectively, P = .8), the similar area under the receiver-operator characteristic curve (0.641-0.690, 0.631-0.698, 0.643-0.654, and 0.678-0.698, respectively, P = .7), and a similar decrease in the risk of PTD of 5-10% for each additional millimeter of CL. The optimal cutoff of CL, however, was affected by GA at presentation, so that a higher cutoff of CL was needed to achieve a target negative predictive value for delivery within 14 days from presentation for women who presented later in pregnancy. The optimal thresholds to maximize the negative predictive value for delivery within 14 days were 36 mm, 32.5 mm, 24 mm and 20.5 mm for women who presented at 32 + 0 to 33 + 6 weeks, 30 + 0 to 31 + 6 weeks, 27 + 0 to 29 + 6 weeks and 24 + 0 to 26 + 6, respectively.

CONCLUSION

CL has modest predictive accuracy in women with threatened PTL, regardless of GA at presentation. However, the optimal cutoff of CL for the purpose of clinical decision making in women with PTL needs to be adjusted based on GA at presentation.

Evaluation of cervical stiffness during pregnancy using semiquantitative ultrasound elastography

OBJECTIVE

To evaluate cervical stiffness during pregnancy using ultrasound-derived elastography, a method used to estimate the average tissue displacement (strain) within a defined region of interest when oscillatory compression is applied.

METHODS

Strain was calculated in two regions of interest, the endocervical canal and the entire cervix, from three anatomical planes of the cervix: mid-sagittal in the plane used for cervical length measurement and in cross-sectional planes located at the internal and external cervical os. Associations between strain values, method of ascertainment and patient characteristics were assessed using linear mixed models to account for within-subject correlation. Inter-rater agreement in defining the degree of cervical stiffness was evaluated in 120 regions of interest acquired by two operators in 20 patients.

RESULTS

A total of 1557 strain estimations were performed in 262 patients at 8-40 weeks of gestation. Adjusting for other sources of variation, (1) cervical tissue strain estimates obtained in the endocervical canal were on average 33% greater than those obtained in the entire cervix; (2) measurements obtained in the cross-sectional plane of the external cervical os and sagittal plane were 45% and 13% greater than those measured in the cross-sectional plane of the internal cervical os, respectively; (3) mean strain rates were 14% and 5% greater among parous women with and without a history of preterm delivery compared with those of nulliparous women, respectively, and were on average 13% greater among women with a cervical length of between 25 and 30 mm compared to those with a cervical length of > 30 mm; and (4) cervical tissue strain was more strongly associated with cervical length than with gestational age.

CONCLUSION

Semiquantitative elastography can be employed to evaluate changes in cervical stiffness during pregnancy.

System-level biomechanical approach for the evaluation of term and preterm pregnancy maintenance

Preterm birth is the primary contributor to perinatal morbidity and mortality, with those born prior to 32 weeks disproportionately contributing compared to those born at 32-37 weeks. Outcomes for babies born prematurely can be devastating. Parturition is recognized as a mechanical process that involves the two processes that are required to initiate labor: rhythmic myometrial contractions and cervical remodeling with subsequent dilation. Studies of parturition tend to separate these two processes rather than evaluate them as a unified system. The mechanical property characterization of the cervix has been primarily performed on isolated cervical tissue, with an implied understanding of the contribution from the uterine corpus. Few studies have evaluated the function of the uterine corpus in the absence of myometrial contractions or in relationship to retaining the fetus. Therefore, the cervical-uterine interaction has largely been neglected in the literature. We suggest that a system-level biomechanical approach is needed to understand pregnancy maintenance. To that end, this paper has two main goals. One goal is to highlight the gaps in current knowledge that need to be addressed in order to develop any comprehensive and clinically relevant models of the system. The second goal is to illustrate the utility of finite element models in understanding pregnancy maintenance of the cervical-uterine system. The paper targets an audience that includes the reproductive biologist/clinician and the engineer/physical scientist interested in biomechanics and the system level behavior of tissues.

Using imaging-based, three-dimensional models of the cervix and uterus for studies of cervical changes during pregnancy

Preterm birth affects over 12% of all pregnancies in the United States for an annual healthcare cost of $26 billion. Preterm birth is a multifactorial disorder but cervical abnormalities are a prominent feature in many patients. Women with a short cervix are known to be at increased risk for preterm birth and a short cervix is used to target therapy to prevent preterm birth. Although the clinical significance of a short cervix is well known, the three-dimensional anatomical changes that lead to cervical shortening are poorly understood. Here, we review our previous studies of the three-dimensional anatomy of the cervix and uterus during pregnancy. The rationale for these studies was to improve our understanding of the deformation mechanisms leading to cervical shortening. Both magnetic resonance imaging and three-dimensional (3D) ultrasound were used to obtain anatomical data in healthy, pregnant volunteers. Solid models were constructed from the 3D imaging data. These solid models were used to create numerical models suitable for biomechanical simulation. Three simulations were studied: cervical funneling, uterine growth, and fundal pressure. These simulations showed that cervical changes are a complex function of the tissue properties of the cervical stroma, the loading conditions associated with pregnancy and the 3D anatomical geometry of the cervix and surrounding structures. An improved understanding of these cervical changes could point to new approaches to prevent undesired cervical shortening. This new insight should lead to therapeutic strategies to delay or prevent preterm birth.

Three-dimensional, extended field-of-view ultrasound method for estimating large strain mechanical properties of the cervix during pregnancy

Cervical shortening and cervical insufficiency contribute to a significant number of preterm births. However, the deformation mechanisms that control how the cervix changes its shape from long and closed to short and dilated are not clear. Investigation of the biomechanical problem is limited by (1) lack of thorough characterization of the three-dimensional anatomical changes associated with cervical deformation and (2) difficulty measuring cervical tissue properties in vivo. The objective of the present study was to explore the feasibility of using three-dimensional ultrasound and fundal pressure to obtain anatomically-accurate numerical models of large-strain cervical deformation during pregnancy and enable noninvasive assessment of cervical-tissue compliance. Healthy subjects (n = 6) and one subject with acute cervical insufficiency in the midtrimester were studied. Extended field-of-view ultrasound images were obtained of the entire uterus and cervix. These images aided construction of anatomically accurate numerical models. Cervical loading was achieved with fundal pressure, which was quantified with a vaginal pressure catheter. In one subject, the anatomical response to fundal pressure was matched by a model-based simulation of the deformation response, thereby deriving the corresponding cervical mechanical properties and showing the feasibility of noninvasive assessment of compliance. The results of this pilot study demonstrate the feasibility of a biomechanical modeling framework for estimating cervical mechanical properties in vivo. An improved understanding of cervical biomechanical function will clarify the pathophysiology of cervical shortening.