To assess the potential impact of the fetus on the characteristics of the labor process, it is important that the obstetrician be knowledgeable of the basic concepts used routinely to describe how the body of the fetus is located in the uterus. The obstetrician should be able to determine the fetal lie, presentation, and position, using the maternal vertebral column and pelvis as reference points, by Leopold's maneuvers, vaginal examination, and if necessary, ultrasound.
Leopold's Maneuvers
Leopold's maneuvers1 consist of an abdominal examination divided into four steps of palpation of the gravid uterus and fetus (Fig. 1). The examiner may be able to palpate the presenting part. In addition, a clinical estimate of the degree of engagement of the presenting part could be made, although the final determination of engagement must be made by way of a vaginal examination. Through use of Leopold's maneuvers, a clinical estimate of the fetal weight also can be obtained, although this is not a formal part of this examination. The accuracy of Leopold's maneuvers can be hampered by the maternal body habitus, the presence of uterine fibroids, multiple gestations, or polyhydramnios. What follows is a description of these maneuvers:
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Maneuver I: The uterine contour is outlined. The fundus is palpated with the fingertips of both hands facing toward the maternal xiphoid cartilage. This should allow the identification of the fetal parts in the upper pole (fundus) of the uterus.
Maneuver II: Once an assessment is made of the fetal part present in the uterine fundus, the hands are placed at either side of the maternal abdomen. With this maneuver, the examiner will be able to determine the location of the fetal back.
Maneuver III: Using one hand, the examiner will grasp the presenting part between the thumb and fingers. This is done on the lower abdomen, a few centimeters above the symphysis pubis. This will allow the examiner to develop a further identification of the presenting part and assessment of its engagement.
Maneuver IV: This last maneuver resembles the first one, but instead of facing the fundus, the examiner faces the pelvis of the patient. The palms of both hands are placed on either side of the lower maternal abdomen, with the fingertips facing toward the pelvic inlet. This maneuver should allow the identification of the fetal parts in the lower pole of the uterus.
After the abdominal examination, a digital vaginal examination is performed. The status of the amniotic membranes and the degree of engagement of the presenting part are indicators of fetal presentation and position. The vaginal examination also allows the clinician to assess the degree of cervical dilation and effacement. If the presenting part is not easily palpable, it is important that further assessment be obtained by performing an ultrasound examination. The ultrasound should be 100% accurate in diagnosing the type of fetal presentation.
Fetal Lie
Using the techniques described above, the clinician should be able to develop an assessment of the relationship between the fetal and maternal dorsal columns (the longitudinal axis of the human body). If the fetus and maternal column are parallel (on the same long axis), the lie is termed vertical or longitudinal lie. This is the most common lie of fetuses in labor. Contrary to this is the transverse lie, in which the fetus is positioned 90° with respect to the mother's dorsal column. There are variants of these two lies, in which the fetus may be in transition from a vertical to a transverse lie. These are the oblique lies.
A fetus can be in an unstable or variable lie when the head is completely unengaged and floating. This situation is seen mostly in cases of severe polyhydramnios and prematurity. The obstetrician should be aware that a fetus in this particular type of lie is susceptible to a cord accident if the patient is in active labor and her cervix is dilating.
Attitude
In addition to having a lie, the fetus has an attitude. This is defined as the relation of the various parts of the fetus to each other. In the normal attitude, the fetus is in universal flexion. The anatomic explanation for this posture is that it enables the fetus to occupy the least amount of space in the intrauterine cavity. The fetal attitude is extremely difficult, if not impossible, to assess without the help of an ultrasound examination.
Presentation
After the lie of the fetus is assessed, the clinician has to detail the fetus further by describing the lowermost structure of the fetus in the maternal pelvis. This is referred to as the fetal presentation. In a vertical (or longitudinal) lie, the fetal presentation can be either cephalic or breech. In the transverse lie, the presentation is usually the back or shoulder; in the oblique lie, it is usually the shoulder or the arm.
The cephalic presentation can be further categorized based on the degree of flexion of the fetal head: A well-flexed head is described as a vertex presentation, an incomplete flexion as a sinciput presentation, a partially extended (deflexed) head as a brow presentation, and a complete extension of the head as a face presentation.
Breech presentation can be categorized on the basis of the attitude or flexion of the hip and knee joints. If there is flexion at the hip and extension at the knees, the fetus is a frank breech. If there is flexion at both the hip and knee joints, the fetus is a full or complete breech. A footling breech has one or both hips and knees in a partial or intermediate extension; this fetus is sometimes called an incomplete breech.
Presentations other than cephalic or breech in a singleton pregnancy require an abdominal route of delivery. These presentations are uncommon, occurring in less than 1% to 2% of all deliveries. Abnormal presentations occur more often in cases of multiple gestation, usually affecting the second twin. Depending on the clinical condition, vaginal delivery of a malpresenting second twin is possible.
Position
The next step in the assessment of the fetus consists of determining the position of the presenting part. This is a description of the relation of the presenting part of the fetus to the maternal pelvis.
In the case of a longitudinal lie with a vertex presentation, the occiput of the fetal calvarium is the landmark used to describe the position. When the occiput is facing the maternal pubic symphysis, the position is termed direct occiput anterior. If the occiput is between the ischial spines and the symphysis, it is called either a right or left occiput anterior. If the occiput is located halfway between the promontorium of the sacrum and the symphysis, the position is termed either a left or right occiput transverse. As the occiput approaches the sacrum, it becomes either a right or left occiput posterior. When the occiput is straight down (i.e., facing the sacrum or coccyx), the position is called direct occiput posterior. This method of describing the fetal position can be applied to other presentations by substitution of the vertex for the presenting fetal anatomic landmark. In cases of breech presentation, the fetal sacrum is used for position.
With transverse and oblique lies, the shoulder structures (acromion) can be used for the description of position. The incidence of transverse lie has been reported as 0.3%.2 Fetal malformations can cause these (and other) malpresentations. These two types of lies have been associated with grandparity, contracted maternal pelvis, placenta previa, prematurity, polyhydramnios, and uterine abnormalities such as transverse septum or leiomyomas. A patient in her last month of pregnancy with a fetus in a transverse or oblique lie would require an ultrasound and physical examination for the evaluation of any of the above risk factors. If no contraindications are identified, one possible management alternative would be an external cephalic version. The success of external cephalic version in cases of transverse lie has been reported as high as 83%,3 even when performed intrapartum. If an elective cesarean section is to be performed, it should be scheduled for the last week of the pregnancy, because 83% of transverse lies will spontaneously convert to a vertex presentation before 39 weeks.4 This expectant management has to be individualized. Patients showing signs of early labor or those with cervical dilation are candidates for an attempt of external version or delivery by cesarean section before 39 weeks.
The fetal anatomic landmarks used to describe the position in the brow and face presentations are the bregma and the mentum. The outlook of the persistent brow presentation for vaginal delivery is poor. Approximately two thirds of brow presentations will convert to vertex or face.2 Fortunately, this is a rare presentation, with an incidence of only 0.05%.5 If the presentation persists as a brow, a cesarean section should be performed. The incidence of face presentation has been quoted as 0.21% of all deliveries.6 In cases of face presentation, a vaginal delivery can be accomplished most frequently with a mentum anterior position, but the mentum posterior position will impede the fifth cardinal movement of labor (extension). Most fetuses in face or mentum posterior position that do deliver vaginally do so after converting during the internal rotation into a mentum anterior or mentum transverse position.
A fetus in a persistent mentum posterior position could be delivered vaginally if it were very premature or the maternal pelvis large. Most frequently, this will not be the case. More than 75% of fetuses at term with mentum posterior position will require delivery by cesarean section because of labor dystocia. This contrasts with a more than 88% success rate of vaginal delivery with the mentum anterior position.7 In view of these data, many clinicians opt to perform a cesarean section for fetuses with a face presentation and a mentum posterior position, without allowing for a trial of labor. Even if vaginal delivery of a mentum anterior presentation is attempted, the diameter of the presenting part may exceed the maternal pelvic capacity. A situation like this also would require an abdominal delivery. Before allowing vaginal delivery of a fetus in a face presentation and a mentum anterior position, the clinician should exclude the possibility of a fetal malformation causing the malpresentation. The brow and face presentation should alert the clinician to the possibility of a congenital fetal anomaly, such as thyromegaly, cystic hygroma, anencephaly, encephalocele, cervical neural tube defects, and teratomas, that could be preventing the flexion of the head. In addition, maternal factors, such as preterm labor, abnormal pelvis, multiple gestation, and a low-lying placenta, have been associated with deflexion of the fetal head. After excluding the possibility of a fetal malformation with the use of a detailed ultrasound examination, the clinician should then exclude the possibility of a contracted maternal pelvis. If on clinical pelvimetry there are any signs of an abnormal pelvis, the clinician should consider obtaining an x-ray or digital pelvimetry.
In addition to the above presentations and fetal positions, there are compound presentations in which one or more extremities of the fetus will present in addition to the vertex or breech. This has been described as “prolapse of an extremity alongside of the presenting part, both entering the pelvic canal simultaneously.”8 The incidence of this type of presentation has been reported to range from 0.15%9 to 0.4%.10 The most frequent is a vertex and hand combination, which accounts for 58% of the cases.9 This is followed in incidence by a vertex, hand, and umbilical cord combination (17%). It is extremely important that the clinician exclude the presence of an umbilical cord prolapse when evaluating a patient with a compound presentation. Risk factors for this type of presentation include contracted maternal pelvis, preterm labor, small for gestational age fetuses, and grandparity. The majority of women whose fetuses have compound presentations can be delivered vaginally. In a series of 131 patients,10 only 2 required a cesarean section. During the course of labor, the extremity will usually slide to the side of the body and the compound presentation will spontaneously convert to a vertex presentation.
Station
In addition to the fetal lie, presentation, and position, the level or station of the presenting part in the maternal pelvis is an important factor in the labor process. For this assessment, the ischial tuberosities of the maternal pelvis are used as the anatomic landmark structures. If the presenting part is above the spines, it is described by the use of negative numbers. If the presenting part is nonpalpable, it is in a floating station; if it is well applied against the cervix, but above the spines, it can range from -1 to -3. There is a significant subjective variation among examiners in the assignment of station. Some consider these numbers to be equivalent to centimeters. Once a presenting part is at the level of the spines, it is considered to be engaged and at 0 station. Below the 0 station, positive numbers are used. If the presenting part is against the pelvic floor, it is at +3 station.
Cardinal Movements of Labor
To accommodate itself to the maternal pelvic dimensions, the fetus must undergo a series of changes in the attitude of its presenting part. This is required for fetal descent through the birth canal. The cardinal movements of labor in a vertex presentation are as follows:
- Engagement: The vertex is engaged when the biparietal diameter is at the level of
the pelvic inlet or lower. In clinical practice, this translates into
an occiput palpable at 0 station.
- Descent: This cardinal movement will usually happen concomitant with engagement, and
is typically documented late in the active phase of the first stage
of labor.
- Flexion: As the vertex descends into the maternal pelvis, it will encounter resistance
from the maternal pelvic floor muscles. This will cause the flexion
of the fetal head onto the thorax.
- Internal rotation: At the moment of engagement and descent of the vertex into the pelvis, the
fetus will be in a transverse position. Because of the anatomic configuration
of the pubococcygeus and iliococcygeus muscles, the occiput
will be forced to rotate to the symphysis pubis. This is the widest
area of the pelvic floor that will allow the passage of the fetus.
- Extension: At the moment of the delivery of the fetal head, the combined effects
of the uterine contractions and the pelvic floor result in this cardinal
movement.
- External rotation: This movement occurs as a consequence of the alignment of the head of
the fetus with its spine as the pressure from the maternal pelvis and
muscles on the fetal head is alleviated.
- Expulsion: After delivery of the head, the anterior shoulder will descend under the
pubic bone. As it is delivered, it will follow the external rotation
of the head of the fetus.
It is important for the reader to understand that these are not separate events. They are all closely related, and some will happen simultaneously.
Concomitant to the cardinal movements, there can be significant molding of the presenting part. This complex process can be deeply influenced by a large number of factors. Abnormalities of the shape and size of the presenting part or maternal pelvis can cause problems affecting not only the capacity of the fetus to descend, but also its internal rotation. Most fetuses initiate the cardinal movements in a occiput transverse position. If its internal rotation were to be disrupted, the fetus could present in an occiput posterior position. Internal rotation can be affected by the administration of epidural anesthesia. If the pelvic floor musculature is suddenly relaxed by a strong epidural block, internal rotation could be directed toward the maternal sacrum, rather than toward the symphysis. In addition, the head of the fetus could become asynclitic. This condition occurs when the head is not in the same longitudinal axis as the fetal vertebral column. The gross appearance of an asynclitic head is that of a lateral tilt toward the shoulder. The major problem of asynclitism is that it causes the head to present a larger diameter to the maternal pelvis than what it would otherwise present. This could lead to labor dystocia due to a relative cephalopelvic disproportion.
Additional problems related to the sudden relaxation of the pelvic floor muscles include possible lack of flexion of the presenting part. In a fetus in a vertex lie, this could result in a brow or face presentation.
Other factors that could influence the position of the presenting part include the maternal position during labor, the presence of fetal or uterine anomalies, and the type of maternal pelvis. In Western society, it has been a common practice for women to labor on their back or side. Midwives frequently have their patients labor in positions other than the back or side so that the occiput on posterior position is prevented, although this practice has not been studied objectively.
