![]() While engagement is an essential element of the cardinal movements, it can, and often does, happen before labor starts. Engagement of the fetal head is confirmed when the biparietal diameter of the vertex is at or past the level of the pelvic inlet. Station of the fetal head refers to where the biparietal diameter is in relation to the ischial spines of the maternal pelvis. Labor and Birth Just the facts In this chapter, youll learn: types of fetal presentations and positions ways in which labor can be stimulated signs and symptoms of labor stages and cardinal movements of labor nursing responsibilities during labor and birth, including ways to provide comfort and support. The mechanical steps of the fetus in its passage through the birth canal are engagement, descent, flexion, internal rotation (most commonly, this is where the fetus rotates to the occiput-anterior position), extension, external rotation, and expulsion ( Fig. 1 The pear shape of the uterus, as well as the gynecoid shape of the maternal pelvic bones, positions the fetus in the vertex presentation in nearly 97% of deliveries. ![]() Recent studies have posited that the surfactant molecule in the lungs of the fetus may play a role in parturition. Following the onset of early labor, prostaglandins E2/F2-alpha, estrogen, and oxytocin coordinate to stimulate the myometrium, providing regular, rhythmic uterine contractions. As the uterus readies for labor over the course of days and weeks, the increased expression of prostaglandin and oxytocin receptors is upregulated, in addition to an increase in myometrial gap junction formation. Throughout pregnancy, true labor is held in check by inhibitors such as progesterone, prostaglandin I-2, relaxin, and nitrous oxide. The physiology of parturition is an orchestrated event involving many players-the myometrium, the decidua, and the uterine cervix.
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