Each of these fetal station values represents the number of centimeters above or below the level of the ischial spines. In between, -5, 0, and +5, fetal stations are assigned sequential values as shown here. The ischial spines are assigned a fetal station value of zero. The significance of the ischial spines is that they represent the narrowest part of the birth canal, and therefore, the most difficult portion of the canal for the fetal head and shoulders to traverse. Rotating to a view from above the pelvis gives us an even better look. Let’s switch to a frontal view to clarify their anatomical position. The ischial spines are difficult to depict on a cross-sectional side view. Located approximately halfway between the pelvic inlet and the pelvic outlet are the ischial spines of the maternal pelvis. There are some variations, but the most commonly used fetal station system today assigns a value of -5 to the pelvic inlet and a value of +5 to the pelvic outlet. In order to chart the position of the fetus as it moves through the birth canal, a system of fetal stations was devised many years ago and is still in use today. The orientation of the long axis of the pelvic inlet and pelvic outlet will come into play when we examine the cardinal movements later in the presentation. The long axis goes from the front to the back, the pelvic outlet is deeper than it is wide. In other words, the pelvic inlet is wider than it is deep.Ĭhanging our view to the bottom of the pelvis, we can see that the situation for the pelvic outlet is the opposite of that of the inlet. The long axis of the pelvic inlet goes from left to right. Viewing the pelvis from above, the yellow line highlights the outer border of the pelvic inlet. In relation to how the cardinal movements facilitate passage of the fetus through the birth canal, it is important to note one key anatomical difference between the pelvic inlet and the pelvic outlet. The fetus enters the birth canal at the pelvic inlet and exits through the pelvic outlet. The pelvic outlet is described by a line going from the bottom of the pubic bone to the tip of the sacrum. The pelvic inlet is described by a line going from the top of the pubic bone to the top of the sacrum. While the lowest part of the mother’s spine, the sacrum makes up the back (posterior) wall. Viewing the fetus and the maternal pelvis in a cross-sectional view from the mother’s right side, the front (also known as the anterior) border of the birth canal is provided by the mother’s pubic bone. To better understand the significance of the cardinal movements, let’s do a brief review of maternal pelvic anatomy. The cardinal movements in labor are so named because they are considered essential to successful navigation by the fetus of the maternal birth canal. Cal Shipley with a review of the seven cardinal movements in labor.
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