The lie is longitudinal, the presentation is cephalic, the presenting part is the face, the attitude is one of complete extension, the chin (mentum, M) is the denominator and leading pole, and the presenting diameter is the submentobregmatic of 9.5 cm. In face presentations, the part between the glabella and chin presents; in brow presentations, it is the part between the glabella and bregma. However, positions intermediate to these are seen.
The incidence is less than 1 percent (one in 600-800) and is higher in multiparas than primigravidas. Primary face presentations are present before the onset of labor and are rare. Most face presentations are secondary, extension taking place during labor generally at the pelvic inlet. About 70 percent of face presentations are anterior or transverse, while 30 percent are posterior.
Anything that delays engagement in flexion can contribute to the etiology of attitudes of extension. There is an association between attitudes of extension and cephalopelvic disproportion, and since this is a serious combination, the presence of a small pelvis or a large head must be ruled out with certainty. Prematurity is another etiology; as with smaller head dimensions, preterm infants can engage before conversion to vertex position. Rare causes of extension include thyroid neoplasms, which act by pushing the head back; multiple coils of cord around the neck, which prevent flexion; and spasm or shortening of the extensor muscles of the neck. Anencephalic fetuses frequently present by the face. In many cases, no cause can be found.
Anterior Face Presentations
The following descriptions apply to the left mentum anterior (LMA) presentation. The mechanism for the right mentum anterior (RMA) presentation is similar to that for LMA except that the chin, small parts, and fetal heart are on the right side, and the back and cephalic prominence are on the left.
Diagnosis of Position: LMA
The long axes of the fetus and mother are parallel (Fig. 15-17)
The head is at the pelvis. Early in labor, the head is not engaged
The back is on the right side of the mother’s abdomen, but since it is posterior, it is often felt indistinctly. The small parts are on the left and anterior. Extension of the spine causes the chest to be thrown out and the back to be hollowed
The breech is in the fundus
The cephalic prominence (the occiput) is on the right. An important diagnostic sign of extension attitudes is that the back and the cephalic prominence are on the same side. When flexion is present, the cephalic prominence and the back are on opposite sides
It must be kept in mind that in anterior face presentations, the baby’s back and occiput are posterior. When the chin is posterior, on the other hand, the back and occiput are anterior
The fetal heart tones are transmitted through the anterior chest wall of the fetus and are heard loudest in the left lower quadrant of the maternal abdomen on the same side as the small parts.
The clue to diagnosis is a negative finding—that is, absence of the round, even, hard vertex. In place of the dome of the skull with its identifying suture lines and fontanels, there is a softer and irregular presenting part. One suspects a face or breech presentation. Identification of the various parts of the face clinches the diagnosis. After prolonged labor, marked edema may confuse the picture
The long axis of the face is in the right oblique diameter of the pelvis (Fig. 15-17B)
The chin is in the left anterior quadrant of the maternal pelvis
The forehead is in the right posterior quadrant of the pelvis
Vaginal examination must be performed gently to avoid injury to the eyes
Ultrasonography can be useful for radiographic demonstration of the hyperextended head with the facial bones at or below the pelvic inlet
Because most face presentations make good progress, the diagnosis may not be made until the face has reached the floor of the pelvis or until advance has ceased.
For some reason, the head does not flex. Instead, it extends (Fig. 15-18), so that in place of an LOP or ROP, there is an RMA or an LMA. The baby enters the pelvis chin first. The presenting diameter in face presentations (submentobregmatic) and in well-flexed head presentations (suboccipitobregmatic) is 9.5 cm in each case. This is one of the reasons why most anterior face presentations come to spontaneous delivery.
A to D. Mechanism of labor. E to G. Birth of the face and head by flexion. H and I. Head falls back in extension. J and K. Restitution and external rotation. LMA, left mentum anterior; LMT, left mentum transverse; MA, mentum anterior.
With the chin as the leading part, engagement takes place in the right oblique diameter of the pelvis. Descent is slower than in flexed attitudes. The face is low in the pelvis before the biparietal diameter has passed the brim. When the forward leading edge of the presenting face is felt at the level of the ischial spines, the tracheobregmatic diameter is still above the inlet.
With descent and molding, the chin reaches the pelvic floor, where it is directed downward, forward, and medially. As it rotates 45° anteriorly toward the symphysis (LMA to mentum anterior [MA]), the long axis of the face comes into the anteroposterior diameter of the pelvis (Figs. 15-18C and D). With further descent, the chin escapes under the symphysis. The shoulders have remained in the oblique diameter, so the neck is twisted 45°. An essential feature of internal rotation is that the chin must rotate anteriorly and under the symphysis, or spontaneous delivery is impossible. Anterior rotation does not take place until the face is well applied to the pelvic floor and may be delayed until late in labor. The attendant must not give up hope too soon.
The head is born by flexion (Figs. 15-18E to G). The submental region at the neck impinges under the symphysis pubis. With the head pivoting around this point, the mouth, nose, orbits, forehead, vertex, and occiput are born over the perineum by flexion. The head then falls back (Figs. 15-18H and I).
As the head is released from the vagina, the neck untwists, and the chin turns 45° back toward the original side (Fig. 15-18J).
The anterior shoulder reaches the pelvic floor and rotates toward the symphysis to bring the bisacromial diameter from the oblique to the anteroposterior diameter of the outlet. The chin rotates back another 45° to maintain the head in its correct relationship to the shoulders (Fig. 15-18K).
Molding (Fig. 15-19) leads to an elongation of the head in its anteroposterior diameter and flattening from above downward. The forehead and occiput protrude. The extension of the head on the trunk disappears after a few days.
Molding: face presentation.
Prognosis: Anterior Face Presentations
Because the face is a poor dilator and because attitudes of extension are less favorable, labor takes longer than in normal occipitoanterior positions. The labor is conducted with this in mind. Delay takes place at the inlet, but when the face presentation and the labor are well established, steady progress is the rule. More than 90 percent of anterior face presentations deliver per vagina without complications. Figure 15-20 summarizes the mechanism of labor with the LMA presentation.
Summary of mechanism of labor: left mentum anterior (LMA). LMT, left mentum transverse; MA, mentum anterior.
The mother has more work to do, has more pain, and receives greater lacerations than in normal positions.
The baby does well in most cases, but the prognosis is less favorable than in normal presentations. The outlook for the child can be improved by early diagnosis, carefully conducted first and second stages of labor, and the restriction of operative vaginal deliveries to easily performed procedures. Cesarean section is preferable to complicated, difficult, and traumatic assisted vaginal deliveries. The membranes rupture early in labor, and the face takes the brunt of the punishment so that it becomes badly swollen and misshapen. Its appearance is a great worry to the parents. The edema disappears gradually, and the infant takes on a more normal appearance. Edema of the larynx may result from prolonged pressure of the hyoid region of the neck against the pubic bone. For the first 24 hours, the baby must be watched carefully to detect any difficulty in breathing.
Management of Anterior Face Presentations
Disproportion: Disproportion is managed by cesarean section
Normal pelvis: In a normal pelvis, anterior face presentations are left alone for these reasons:
Most deliver spontaneously or with the aid of low forceps
If conversion (flexion) is successful, the anterior face presentation is replaced by an occipitoposterior one (LMA to ROP or RMA to LOP). This does not improve the situation and may make it worse
If conversion is partially successful, the face is changed to a brow presentation. In this case, a face presentation, which usually delivers spontaneously, is replaced by a brow presentation, which cannot
Low in the pelvis, well below the ischial spines: Extraction with low forceps
High in the pelvis: Cesarean section
Transverse Face Presentations
The long axis of the face is in the transverse diameter of the pelvis, with the chin on one side and the forehead on the other (Fig. 15-21).
The following descriptions apply to the left mentum transverse (LMT) presentation. The mechanism of labor for the right mentum transverse (RMT) presentation is the same as that for LMT except that the chin, small parts, and fetal heart are on the right, and the back and cephalic prominence are on the left.
Diagnosis of Position: LMT
The long axis of the fetus is parallel to that of the mother
The head is at the pelvis
The back is on the right, toward the maternal flank. The small parts are on the left side
The breech is in the fundus
The cephalic prominence (the occiput) is on the right, the same side as the back
The fetal heart is heard loudest in the left lower quadrant of the mother’s abdomen.
The long axis of the face is in the transverse diameter of the pelvis
The chin is to the left at 3 o’clock
The forehead is to the right at 9 o’clock
A summary of the mechanism of labor for the LMT presentation is given in Figure 15-22.
Mechanism of labor: left mentum transverse (LMT). LMA, left mentum anterior; MA, mentum anterior.
Extension to LMT occurs instead of flexion to ROT.
Engagement takes place in the transverse diameter of the pelvis. Descent is slow.
The chin rotates 90° anteriorly to the midline (LMT to LMA to MA). The chin comes under the symphysis.
The submental region of the neck impinges in the subpubic angle. Birth is by flexion, after which the head falls backward.
As the neck untwists, the head turns back 45°.
The shoulders turn from the oblique into the anteroposterior diameter of the pelvis, and the head rotates back another 45°.
Clinical Course of Labor and Management: LMT
Anterior rotation takes place in the majority of cases, LMT to LMA to MA. The treatment is the same as LMA. Delivery is spontaneous or assisted by low forceps
Arrest as LMT low in the pelvis
Rotation to LMA manually or by forceps followed by extraction of the head by forceps
If rotation is difficult or fails, cesarean section is performed
Arrest as LMT high in the pelvis is treated by cesarean section
Posterior Face Presentations
Some 30 percent of face presentations are posterior. Most of these rotate anteriorly. The flexed counterpart of the posterior face is the anterior occiput; thus, LMP flexes to ROA and RMP to LOA. Persistent posterior face presentations become arrested because they cannot deliver spontaneously. The descriptions here are for the left mentum posterior (LMP) presentation.
Diagnosis of Position: LMP
The long axis of the fetus is parallel to the long axis of the mother (Fig. 15-23)
The head is at the pelvis
The back is anterior and to the right. The small parts are on the left and posterior
The breech is in the fundus of the uterus
The cephalic prominence (occiput) is to the right and anterior. It is on the same side as the back
The fetal heart tones, transmitted through the anterior shoulder, are heard loudest in the left lower quadrant of the mother’s abdomen.
The long diameter of the face is in the left oblique diameter of the pelvis
The chin is in the left posterior quadrant of the pelvis (Fig. 15-23B)
The forehead is in the right anterior quadrant
There are two basic mechanisms:
Long arc rotation, with the chin rotating 135° to the anterior. About two-thirds of posterior face presentations do this and deliver spontaneously or with the aid of low forceps
Short arc rotation of 45° to the posterior, with the chin ending up in the hollow of the sacrum. These cases become arrested as persistent posterior face presentations
Long Arc Rotation: 135° to the Anterior
Extension to LMP (Fig. 15-24) occurs instead of flexion to ROA.
Left mentum posterior (LMP): long arc rotation. LMA, left mentum anterior; LMT, left mentum transverse; MA, mentum anterior.
Descent is slow. The presenting part remains high while the essential molding takes place. Without extreme molding, the vertex cannot pass under the anterior part of the pelvic inlet.
The slow descent continues; the marked molding enables the chin to reach the pelvic floor, where it rotates 135° to the anterior and comes to lie under the symphysis. Since the original position was LMP, the sequence is LMP to LMT to LMA to MA in rotations of 45° between each step (Figure 15-24B to D).
The submental area pivots under the symphysis, and the head is born by flexion. The head then falls backward.
The chin rotates back 45° as the neck untwists.
With the rotation of the shoulders from the oblique into the anteroposterior diameter of the pelvis, the chin turns back another 45°.
Short Arc Rotation: 45° to the Posterior
Extension to LMP takes place (Fig. 15-25).
Left mentum posterior (LMP): short arc rotation. MP, mentum posterior.
Descent occurs with the help of extreme molding.
The chin rotates 45° posteriorly into the hollow of the sacrum (LMP to MP). Impaction follows, and the progress of labor comes to a halt. Flexion cannot take place and further advancement is not possible, except in the rare situation in which the baby is so small that the shoulders and head can enter the pelvis together.
Prognosis: Posterior Face Presentations
The prolonged labor and difficult rotation are traumatic to both the baby and mother. When the chin rotates posteriorly, the prognosis is poor unless the situation is corrected. Maternal morbidity is directly proportional to the degree of difficulty of the birth. High forceps or version and extraction carry with them the most morbid postpartum courses.
Management of Posterior Face Presentations
Disproportion: Disproportion is managed by cesarean section
Trial of labor: Since two-thirds of posterior faces rotate anteriorly and deliver spontaneously, and since internal rotation may not be completed until later in labor when the face is distending the pelvic floor, plenty of time should be allowed for the rotation to be accomplished. Interference must not be premature
Persistent posterior face: Since face presentations that have remained posterior cannot be delivered spontaneously, operative delivery is necessary
Cesarean section is the modern treatment of choice, giving the best results for both the mother and child
Flexion (conversion) from mentoposterior to occipitoanterior may be considered if cesarean section cannot be performed. One method of accomplishing this is by the Thorn maneuver (Fig. 15-26). The cervix must be fully dilated. With the vaginal hand, the operator flexes the fetal head. With the other hand, the operator pushes on the breech to flex the body. At the same time, an assistant presses against the baby’s thorax or abdomen to try and jack-knife the infant’s body. This procedure is performed under anesthesia and must be done soon after the membranes rupture. If the amniotic fluid has drained away, the dry uterine cavity and snug fit of the uterus around the baby make it difficult or impossible to carry out this treatment. Once flexion has been accomplished, the head is pushed into the pelvis and held in place
Rotation to mentum anterior can sometimes be achieved by the use of forceps, but the operation is difficult and may be traumatic
In the setting of a normal pelvis and effective contractions, successful vaginal delivery with face presentation is usually possible. It is important to monitor fetal heart rate with external devices because internal monitoring may cause injury to the face and eyes. In the presence of pelvic inlet contraction and posterior face presentation, cesarean delivery is usually indicated.