Phases of second stage labor. Both rely on careful regulation of smooth muscle contraction. This permits an increasing portion of the uterine contents to occupy the lower segment. In this circumstance, the placenta descends sideways, and its maternal surface appears first. Concurrently, the uterus must initiate extensive changes in its size and vascularity to accommodate the pregnancy and prepare for uterine contractions. Immediately and for about an hour or so after delivery, the myometrium remains in a state of rigid and persistent contraction and retraction. National Center for Biotechnology Information, Unable to load your collection due to an error, Unable to load your delegates due to an error. After the cervix is dilated fully, the most important force in fetal expulsion is that produced by maternal intraabdominal pressure. 1. This is typically augmented by uterotonics (Chap. 1 Introduction. Author information: (1)Presbyterian/St. FIGURE 21-11 Uterine myocyte relaxation and contraction. For the placenta to accommodate itself to this reduced area, it increases in thickness, but because of limited placental elasticity, it is forced to buckle. Changes in Uterine Shape During Labor. Hyaluronidase genes are expressed in both the mouse and human cervix, and increased hyaluronidase activity is reported in the mouse cervix at term (Akgul, 2012). 21-4). 21-3). The passive lower uterine segment is derived from the isthmus, and the physiological retraction ring develops at the junction of the upper and lower uterine segments. Membranes usually remain in situ until placental separation is nearly completed. Actin now assumes a fibrillar form, and calcium enters the cell to combine with calmodulin to form complexes. Specifically, collagen processing and the number or type of covalent cross-links between collagen triple helices are altered. These result from altered extracellular matrix structure or composition (Lowder, 2007; Rahn, 2008).   Privacy The curve is based on analysis of data derived from a large, nearly consecutive series of women. USA.gov. These anatomical structures are shown in detail in Chapter 2 (p. 22). There are no therapies to prevent premature cervical ripening. These protect the reproductive tract from invasion by commensal microorganisms and restore endometrial responsiveness to normal hormonal cyclicity. Calcium binds to calmodulin, a calcium-binding regulatory protein, which in turn binds to and activates myosin light-chain kinase. R.R. The marked changes within the extracellular matrix during cervical ripening in phase 2 are accompanied by stromal invasion with inflammatory cells. In active-phase labor, the duration of each contraction ranges from 30 to 90 seconds, averaging about 1 minute. To describe the physiology underlying the three stages of labour, including the physiology of effective uterine contractions, the mechanism of the second stage and the steps involved in placental separation in the third stage. The duration of the latent phase is more variable and sensitive to changes by extraneous factors. Changes in the amount of core protein or in the number, length, or degree of sulfation of GAG chains can influence proteoglycan function. As effacement begins, the multiparous cervix shows dilatation and funneling of the internal os. Preterm cervical dilatation, structural incompetence, or both may forecast delivery (Iams, 1996). Clinically, the maintenance of cervical anatomical and structural integrity is essential for continuation of pregnancy to term. Once the second stage commences, only progressive descent of the presenting part will foretell further progress. But, others suggest that the biomechanical changes are not consistent solely with collagenase activation and loss of collagen. The transition from the softening to the ripening phase begins weeks or days before onset of contractions. The anatomical second stage of labour has been traditionally defined as the period from full dilatation of the os uteri to the birth of the baby. The interaction of myosin and actin is essential to muscle contraction. This plexiform arrangement aids greater shortening and force-generating capacity. At this time, the speed of descent is also maximal and is maintained until the presenting part reaches the perineal floor (Friedman, 1978). Cervical modifications during this second phase principally involve connective tissue changes—so-called cervical ripening. For example, Buhmschi and colleagues (2004) performed tissue biomechanical studies in the rat and suggest that ripening correlates with changes in the three-dimensional structure of collagen rather than its degradation by collagenases. These forces thus can be brought to bear irrespective of the fetal lie or presentation. Hegar (1895) first described palpable softening of the lower uterine segment at 4 to 6 weeks’ gestation, and this sign was once used to diagnose pregnancy.

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