Chapter 10: Normal Mechanisms of Labor

LEFT OCCIPUT ANTERIOR: LOA

LOA is a common longitudinal cephalic presentation (Fig. 10-1). Two-thirds of occiput anterior positions are in the LOA position. The attitude is flexion, the presenting part is the posterior part of the vertex and the posterior fontanelle, and the denominator is the occiput (O).

Diagnosis of Position: LOA

Abdominal Examination

1. The lie is longitudinal. The long axis of the fetus is parallel to the long axis of the mother

2. The head is at or in the pelvis

3. The back is on the left and anterior and is palpated easily except in obese women

4. The small parts are on the right and are not felt clearly

5. The breech is in the fundus of the uterus

6. The cephalic prominence (in this case the forehead) is on the right. When the attitude is flexion, the cephalic prominence and the back are on opposite sides. The reverse is true in attitudes of extension

Fetal Heart

The fetal heart is heard loudest in the left lower quadrant of the mother’s abdomen. In attitudes of flexion, the fetal heart rate is transmitted through their back. The point of maximum intensity varies with the degree of rotation. As the fetal back approaches the midline of the maternal abdomen, so does the point where the fetal heart is heard most strongly. Therefore, in a left anterior position, it is heard below the umbilicus and somewhere to the left of the midline, depending on the exact situation of the back.

Vaginal Examination

1. The station of the head is noted—whether it is above, at, or below the ischial spines

2. If the cervix is dilated, the suture lines and the fontanelles of the fetal head can be felt. In the LOA position, the sagittal suture is in the right oblique diameter of the pelvis

3. The small posterior fontanelle is anterior and to the mother’s left

4. The bregma is posterior and to the right

5. Since the head is probably flexed, the occiput is slightly lower than the brow

Normal Mechanism of Labor: LOA

The mechanism of labor as we know it today was first described by William Smellie during the 18th century. It is the way the baby adapts itself to and passes through the maternal pelvis. There are six movements, with considerable overlap:

1. Descent

2. Flexion

3. Internal rotation

4. Extension

5. Restitution

6. External rotation

The following description relates to left anterior positioning of the occiput.

Descent

Descent, which includes engagement in the right oblique diameter of the pelvis, continues throughout normal labor as the baby passes through the birth canal. The ...