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The Emergency Physician will, on occasion, be required to handle the delivery of an infant when an Obstetrician or Family Physician is not available. The management of normal labor and delivery requires a basic understanding of the mechanisms of labor, the assessment and treatment of the mother, the safe delivery of the infant, and careful observation of both in the immediate postpartum period.

Labor is defined as repetitive uterine contractions leading to cervical change (dilation and effacement). The mechanisms of labor, also known as the cardinal movements of labor, describe the changes in the position of the fetal head as it travels through the birth canal. The safe delivery of the infant is the ultimate goal of labor.

Pelvic Anatomy

A successful vaginal delivery is dependent on the adequacy of the female pelvis. Through the use of clinical pelvimetry, physicians are able to make an assessment if adequate space exists for the passage of the fetus during prenatal visits. In theory, the most useful planes to measure are the pelvic inlet and the midplane. Evaluating the pelvic inlet is done by measuring the diagonal conjugate (Figures 131-1A & B). When assessing the midplane, a measurement is taken of the ischial interspinous or bi-ischial diameter (Figure 131-1C). Inadequate measurements indicate potential problems that may result in fetal entrapment, shoulder dystocia, or prolonged labor.1

Figure 131-1.

Measure pelvic distances to determine if there may be difficulties during the delivery. A. The pelvic conjugate diameters. B. Measuring the diagonal conjugate. C. The ischial interspinous distance.

The diagonal conjugate refers to the distance from the inferior border of the pubic symphysis to the sacral promontory (Figure 131-1A). A normal diagonal conjugate measures approximately 12.5 cm, with the critical distance being 10 cm. To measure the diagonal conjugate place the tip of the middle finger at the sacral promontory and note the point on the hand that contacts the pubic symphysis (Figure 131-1B). The true conjugate, a radiographic measurement of the pelvic inlet, is the distance from the sacral promontory to the superior aspect of the pubic symphysis (Figure 131-1A). However, this measurement cannot be made clinically. The true conjugate can be estimated by subtracting 1.5 to 2 cm from the diagonal conjugate. This distance represents the smallest diameter of the inlet and is normally 11 cm or more (Figure 131-1A). The obstetric conjugate is the distance from the sacral promontory to a point on the inner surface of the pubic symphysis that is a few millimeters from the upper margin of the pubic symphysis (Figure 131-1A). This distance corresponds with the true conjugate and is approximately 11 cm (Figure 131-1A). The critical distance to keep in mind is a ...

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