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      A Comparison of the Contractile Properties of Myometrium from Singleton and Twin Pregnancies

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          Abstract

          Objective

          Over half of twin pregnancies in US and UK deliver prematurely but the reasons for this are unclear. The contractility of myometrium from twin pregnancies has not been directly investigated. The objective of this research was to determine if there are differences in the contractile activity and response to oxytocin, between myometrium from singleton and twin pregnancies, across a range of gestational ages. Furthermore, we wished to determine if contractile activity correlates with increasing level of stretch, using neonatal birth weights as a marker of uterine stretch.

          Methods

          This was an in vitro, laboratory based study of myometrial contractility in women pregnant with one or two babies, using biopsies obtained from non-labouring women undergoing Caesarean section. Spontaneous, oxytocin-stimulated and depolarization induced contractile activity was compared.

          Results

          Direct measurements of myometrial contractility under controlled conditions show that the frequency of contractions and responses to oxytocin are significantly increased in twins compared to singletons. The duration of contraction however was significantly reduced. We find that contractile activity correlates with increasing levels of stretch, using neonatal birth weights as a surrogate for uterine stretch, with response to oxytocin being significantly positively correlated with birth weight.

          Conclusions

          We have found significant differences in contractile properties between myometrium from singleton and twin pregnancies and that increasing uterine stretch can alter the contractile properties of myometrium. We discuss the implication of these findings to preterm delivery and future studies.

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          Most cited references 36

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          Oxytocin receptors in the human uterus during pregnancy and parturition.

          We have determined the concentration and distribution of oxytocin receptors in myometrial and decidual tissues obtained at cesarean section or hysterectomy during pregnancy. Myometrial receptor concentration was low at 13 to 17 weeks but had risen about twelvefold by 37 to 41 weeks. After the onset of labor, either preterm or term, the receptor levels were maximal and significantly higher than before the onset of labor. In cases of failed induction of labor with oxytocin and in postterm pregnancies (43 to 46 weeks), the receptor concentration was significantly lower than in spontaneous labor. Myometrial receptor concentrations in the fundus and the corpus were similar and significantly higher than in the lower part of the uterine segment, and the cervix had the lowest concentration. The parietal decidua had oxytocin receptor concentrations of the same magnitude as the myometrium. These results are consistent with a functional role of endogenous oxytocin in the activation of the human uterus during pregnancy and parturition.
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            Obstructed labour.

            Obstructed labour is an important cause of maternal deaths in communities in which undernutrition in childhood is common resulting in small pelves in women, and in which there is no easy access to functioning health facilities with the capability of carrying out operative deliveries. Obstructed labour also causes significant maternal morbidity in the short term (notably infection) and long term (notably obstetric fistulas). Fetal death from asphyxia is also common. There are differences in the behaviour of the uterus during obstructed labour, depending on whether the woman has delivered previously. The pattern in primigravid women (typically diminishing contractility with risk of infection and fistula) may result from tissue acidosis, whereas in parous women, contractility may be maintained with the risk of uterine rupture. Ultimately, tackling the problem of obstructed labour will require universal adequate nutritional intake from childhood and the ability to access adequately equipped and staffed clinical facilities when problems arise in labour. These seem still rather distant aspirations. In the meantime, strategies should be implemented to encourage early recognition of prolonged labour and appropriate clinical responses. The sequelae of obstructed labour can be an enormous source of human misery and the prevention of obstetric fistulas, and skilled treatment if they do occur, are important priorities in regions where obstructed labour is still common.
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              Insights into the uterus.

               Susan Wray (2007)
              A better understanding of the mechanisms that generate and modulate uterine contractility is needed if progress is to be made in the prevention or treatment of problems in labour. Dysfunctional labour describes the condition when uterine contractility is too poor to dilate the cervix, and it is the leading cause of emergency Caesarean sections. Recently, insight has been gained into a possible causal mechanism for dysfunctional labour. Study of the physiological mechanisms that produce excitation in the uterus, the subsequent Ca(2)(+) signals and biochemical pathway leading to contraction has underpinned this progress. In this review, I give an account of excitation-contraction signalling in the myometrium and explore the implications of recent findings concerning lipid rafts for these processes. I also discuss how changes of pH are fundamentally enmeshed in uterine activity and biochemistry and explore the effect that pH changes will have on human myometrium. Finally, I present the evidence that acidification of the myometrium is correlated with dysfunctional labour and suggest the processes by which it is occurring. It is only by gaining a better understanding of uterine physiology and pathophysiology that progress will be made and research findings translated into clinical benefit for women and their families.
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                Author and article information

                Affiliations
                [1 ]Department of Cellular and Molecular Physiology, Institute of Translational Medicine, University of Liverpool, Liverpool, United Kingdom
                [2 ]Liverpool Women's NHS Foundation Trust, Liverpool, United Kingdom
                [3 ]Department of Women's and Children's Health, Institute of Translational Medicine, University of Liverpool, United Kingdom
                University of Nevada School of Medicine, United States of America
                Author notes

                Competing Interests: The authors have declared that no competing interests exist.

                Contributed to revisions of the paper and provided approval for submission: LB JPN. Conceived and designed the experiments: SA PT LB JPN SW. Performed the experiments: SA PT JP CB. Analyzed the data: SA PT JP CB. Contributed reagents/materials/analysis tools: LB JPN. Wrote the paper: SA PT SW.

                Contributors
                Role: Editor
                Journal
                PLoS One
                PLoS ONE
                plos
                plosone
                PLoS ONE
                Public Library of Science (San Francisco, USA )
                1932-6203
                2013
                6 May 2013
                : 8
                : 5
                23671701 3646038 PONE-D-13-03725 10.1371/journal.pone.0063800

                This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

                Counts
                Pages: 8
                Funding
                SA was funded by SPARKS, UK and LWH Trust R&D. PT was the recipient of a Wolfson bursary. JP was funded by Rosetrees Trust, UK. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
                Categories
                Research Article
                Biology
                Anatomy and Physiology
                Reproductive System
                Reproductive Physiology
                Sexual Reproduction
                Cell Physiology
                Medicine
                Anatomy and Physiology
                Reproductive System
                Reproductive Physiology
                Sexual Reproduction
                Obstetrics and Gynecology
                Pregnancy
                Pregnancy Complications
                Preterm Labor
                Labor and Delivery
                Management of High-Risk Pregnancies
                Women's Health

                Uncategorized

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