The circumference of the fetal hips is usually small enough not to create problems.ĭeviations from the normal attitude may cause difficulties in childbirth. This position creates a shoulder diameter that is smaller than the skull, facilitating passage through the birth canal. Molding can be extensive, but the heads of most newborns assume their normal shape within 3 days after birth.Īlthough the size of the fetal shoulders may affect passage, their position can be altered relatively easily during labor thus one shoulder may occupy a lower level than the other. This capacity of the bones to slide over one another also permits adaptation to the various diameters of the maternal pelvis. Because the bones are not firmly united, however, slight overlapping of the bones, or molding of the shape of the head, occurs during labor. Sutures and fontanels make the skull flexible to accommodate the infant brain, which continues to grow for some time after birth. The posterior fontanel lies at the junction of the sutures of the two parietal bones and the occipital bone, is triangular, and is approximately 1 cm by 2 cm. The larger of these, the anterior fontanel, is diamond shaped, is approximately 3 cm by 2 cm, and lies at the junction of the sagittal, coronal, and frontal sutures. The two most important fontanels are the anterior and posterior ones (see Fig. The information gathered by clinical examination and ultrasound should be integrated into clinical decision making.Ĭardinal movements fetal attitude fetal descent fetal position mechanics of labor ultrasound.Ĭopyright © 2021 The Authors. Ultrasound offers a historically unique opportunity for noninvasive, dynamic studies of the mechanics of labor. We would argue that descent is the main purpose of the uterine powers and cardinal movements, a description of the rotational movements the fetal head and shoulders must perform to obtain descent. German and older English literature lists only 4 rotational movements as the cardinal movements and excludes engagement, descent, and expulsion. Anglo-American literature lists 7 cardinal movements, namely engagement, descent, flexion, internal rotation, extension, external rotation, and expulsion. The fetus negotiates the birth canal and rotational movements are necessary for descent. Electronic address: mechanics of labor describe the forces required for fetal descent, and the movements that the fetus must perform to overcome the resistance met by the maternal bony pelvis and soft tissue. Olavs Hospital, Trondheim University Hospital, Trondheim, Norway (Drs Kahrs and Eggebø) Department of Clinical and Molecular Medicine, Faculty of Medicine and Health Sciences, Norwegian University of Science and Technology, Trondheim, Norway (Drs Kahrs and Eggebø) Department of Obstetrics and Gynecology, Stavanger University Hospital, Stavanger, Norway (Dr Eggebø). 3 National Center for Fetal Medicine, Department of Obstetrics and Gynecology, St.Olavs Hospital, Trondheim University Hospital, Trondheim, Norway (Drs Kahrs and Eggebø) Department of Clinical and Molecular Medicine, Faculty of Medicine and Health Sciences, Norwegian University of Science and Technology, Trondheim, Norway (Drs Kahrs and Eggebø). 2 National Center for Fetal Medicine, Department of Obstetrics and Gynecology, St.1 Department of Obstetrics and Gynaecology, Oslo University Hospital, Oslo, Norway (Dr Iversen) Institute of Clinical Medicine, University of Oslo, Oslo, Norway (Dr Iversen).
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