A forceps is an instrument designed to aid in the delivery of the fetus by applying traction to the fetal head. Many different types of forceps have been described and developed throughout time. Generally, forceps consist of 2 crossing shafts that are maneuvered into a proper relationship with the fetal head and are articulated.
Forceps have 4 major components, as follows:
- Blades: Each blade has 2 curves. The blades are oval or elliptical and can be fenestrated or solid. They have a concave surface and a convex surface, which are applied to the fetal head and the pelvic curvature, respectively.
Shanks: These connect the blades to the handles and provide the length of the device. They are either parallel or crossing.
Lock: Many different types have been designed. The lock is the type of articulation between the shanks.
Handles: These are the location where the operator holds the device and applies traction to the fetal head.
History of the Procedure: The history of obstetrical forceps is long and, often, colorful. Sanskrit writings from approximately 1500 BC contain evidence of single and paired instruments; Egyptian, Greek, Roman, and Persian writings and pictures refer to forceps.
The credit for the invention of the precursor of the modern instruments used on live infants goes to Peter Chamberlain (circa 1600) of England. Modifications have led to more than 700 different types and shapes of forceps. In 1745, William Smellie described the accurate application to the occiput, rather than the previously performed pelvic application, regardless of the position of the head. In 1845, Sir James Simpson designed a forceps that was calculated to appropriately fit both cephalic and pelvic curvatures. In 1920, Joseph DeLee further modified that instrument and presented the concept of prophylactic forceps delivery.
Clinical studies performed before the 1970s suggested that the risk of fetal morbidity and mortality was higher when the second stage of labor exceeded 2 hours .However, using current obstetrical management, morbidity rates no longer increase with longer labors if fetal surveillance is reassuring. Thus, the length of the second stage of labor no longer is an absolute indication for operative termination of labor.
Other factors were at work to decrease the use of forceps deliveries. In particular, the availability of blood products and greater choices in antibiotics helped make cesarean delivery a safe alternative to operative vaginal deliveries.
Frequency: The frequency of operative vaginal deliveries is estimated to be approximately 10% of all vaginal deliveries.
Simpson forceps was the type most commonly used for outlet and low forceps deliveries. Other types of forceps also are available. Their use is even more controversial. Of these, the most common is the Piper forceps, which is used in the delivery of the after-coming head in breech vaginal deliveries. It is designed to decrease traction on the fetal neck during breech delivery. Multiple other types of forceps have been designed to rotate the fetal head or for unusual maternal pelvic shapes. For detailed information on these more unusual forceps procedures, the reader is directed to the excellent book by Dennen, Dennen's Forceps Deliveries.
Clinical: Forceps delivery is classified according to the level and position of the head in the birth canal at the time the forceps are applied. In 1965, the American College of Obstetricians and Gynecologists issued their classification of low and outlet, mid, and high forceps. Each of these could be interpreted differently according to the operator's understanding of the issue. The low and outlet forceps categories were very restrictive and were applied when the fetal scalp was visible, when it had reached the pelvic floor, and when the sagittal suture was in an anteroposterior diameter of the pelvis. The bigger issue was the broad category of mid forceps applications. It included many stations of the fetal head, from engagement at zero station all the way to the perineum.
For these reasons, the American College of Obstetricians and Gynecologists (American College of Obstetricians and Gynecologists, 2000) redefined the classification of station and types of forceps deliveries in 1988. The revised classification uses the level of the leading bony point of the fetal head, in centimeters, measured from the level of the maternal ischial spines, to define station (-5 to 5 cm).
Criteria for types of forceps deliveries
Outlet forceps: (1) The scalp is visible at the introitus, without separating the labia. The fetal skull has reached the pelvic floor. (2) The sagittal suture is in anteroposterior diameter, right or left occiput anterior or posterior position. (3) The fetal head is at or on the perineum. (4) Rotation does not exceed 45°.
Low forceps: The leading point of the fetal skull is at a station greater than or equal to +2 cm and is not on the pelvic floor; any degree of rotation may be present.
Mid forceps: The station is above +2 cm, but the head is engaged.
High forceps: This is not included in the classification.
Obstetric pelvic evaluation and its clinical implications
The important points of interest are emphasized as follows:
For obstetric purposes, the coccygeal mobility and the narrowness of the subpubic arch should be evaluated during the pelvic examination.
The distance between the symphysis pubis and the sacral promontory is measured by digital examination and is defined as the diagonal conjugate.
The obstetrical conjugate, estimated by subtracting 1.5-2 cm from the diagonal conjugate, is important in determining the capability of the presenting part to pass through the pelvic inlet. Unfortunately, no clinical means of direct midpelvic measurement is available. These measurements can only be obtained with detailed imaging studies.
Furthermore, assessing the pelvic outlet is important. The distance between the ischial tuberosities should be at least 8 cm.
During the examination, one may have a high index of suspicion of a contracted mid pelvis if the ischial spines feel quite prominent, the sidewalls are convergent, and the concavity of the sacrum is very shallow. If the biischial diameter of the pelvis is less than 8 cm, a contracted pelvis should be suspected.
For the purpose of assessing a patient for forceps application, the best time to evaluate the pelvis is at the time of delivery, not at the first prenatal visit.
Indications for operative vaginal deliveries are identical for forceps and vacuum extractors. No indication for operative vaginal delivery is absolute.
The following indications apply when no contraindications exist:
- Prolonged second stage: (1) This includes nulliparous woman with failure to descend for 2 hours without, and 3 hours with, conduction anesthesia. (2) It also includes multiparous woman with failure to descend for 1 hour without, and 2 hours with, conduction anesthesia.
Suspicion of immediate or potential fetal compromise is an indication.
Shortening of the second stage for maternal benefits: Maternal indications include, but are not limited to, exhaustion, bleeding, cardiac or pulmonary disease, and history of spontaneous pneumothorax.
In expert hands, fetal malpositions, including the after-coming head in breech vaginal delivery, can be indications for forceps delivery.
Prerequisites for forceps delivery include the following:
- The head must be engaged.
The cervix must be fully dilated and retracted.
The position of the head must be known.
The type of pelvis should be known.
The membranes must be ruptured.
No disproportion should be suspected between the size of the head and the size of the pelvic inlet and mid pelvis.
The patient must have adequate anesthesia.
Adequate facilities and supportive elements should be available.
The operator should be fully competent in the use of the instruments and the recognition and management of potential complications.
An operator should be present who knows when to stop, to not force the issue, and to not aggressively use both forceps and vacuum in combination because this has been shown to increase morbidity for both the mother and fetus.
|RELEVANT ANATOMY AND CONTRAINDICATIONS
Planes and diameters of the pelvis
For obstetrical purposes, the pelvis is described as having 3 imaginary planes, as follows
Plane of the inlet: Four diameters have been described.
- Anteroposterior diameter: This is the distance between the sacral promontory and the symphysis pubis; it is designated the obstetrical conjugate. This conjugate normally measures approximately 10 cm or more, but it may be shortened considerably in an abnormal pelvis.
Transverse diameter: This is the greatest distance between the linea terminalis on either side of the pelvis. This imaginary line usually intersects the obstetrical conjugate at a point approximately 4 cm in front of the promontory.
Two oblique diameters: Each of these diameters extends from one of the sacroiliac joints to the iliopectineal eminence on the opposite side of the pelvis. These diameters normally average less than 13 cm each.
Plane of the mid pelvis: This is the plane of the smallest dimensions. This plane is extremely important following engagement of the head in obstructed labor. The interspinous diameter (approximately >10 cm) usually is the smallest diameter of the pelvis.
Plane of the pelvic outlet: This consists of 2 triangular areas created from the connection of an imaginary line between the 2 ischial tuberosities. The apex of the posterior triangle is at the tip of the sacrum, and the apex of the anterior triangle is under the pubic arch. The following 3 diameters of the outlet are of importance:
- Anteroposterior diameter: This normally is 9.5-11.5 cm and extends from the lower margin of the symphysis pubis to the tip of the sacrum.
Transverse diameter: This commonly is 11 cm and is the distance between the inner edges of the ischial tuberosities.
Posterior sagittal diameter: This usually exceeds 7 cm and extends from the tip of the sacrum to a right-angle intersection with the line between the ischial tuberosities.
Engagement: This occurs when the biparietal diameter in a vertex position passes through the plane of the pelvic inlet. This generally is achieved when the leading bony point of the skull has reached the level of the ischial spines.
Position: This describes the relationship of the fetal presenting part with the maternal pelvis.
Asynclitism: The fetal head accommodates the transverse axis of the pelvic inlet, but the sagittal suture (while remaining parallel to the transverse axis) may be deflected anteriorly or posteriorly and therefore is not exactly between the sacral promontory and symphysis pubis. This is called asynclitism. It can be defined as an anterior or posterior asynclitism.
- For cephalic presentations, the reference point is the occiput, whereas in breech presentations, the reference point is the sacrum.
Position is always described in reference to the maternal right or left side of the pelvis.
Determination of the position is crucial in forceps application and traction. The fontanelles and sutures are used to determine the position. The finding that the fontanelles are not easily palpable is not uncommon; this occurs because of distortion, molding, or caput formation.
The position can be determined by finding the location of the sagittal suture and its relationship to the posterior portion of the ear, if palpable. If the sagittal suture is in a U formation, an anterior asynclitism presentation often occurs (ie, presentation of the anterior parietal bone of the fetal head). Conversely, if the sagittal suture is in the shape of an inverted U, this may indicate posterior asynclitism (ie, posterior parietal bone) presentation.
Most fetuses can be delivered by forceps if they are in or can be maneuvered (manually or by forceps) into an occiput anterior or posterior position.
Correct determination of the position may be the most important step prior to forceps application.
Presentation: This is the relationship between the leading fetal part and the maternal pelvic inlet (eg, cephalic, breech, shoulder).
Lie: This is the relationship between the fetal longitudinal axis and the maternal longitudinal axis (eg, longitudinal, oblique, transverse).
Contraindications: The following are contraindications to forceps-assisted vaginal deliveries:
- Any contraindication to vaginal delivery .
Refusal of the patient to consent to the procedure
Cervix not fully dilated or retracted
Inability to determine the presentation and fetal head position or pelvic adequacy
Suspected cephalopelvic disproportion
Unsuccessful trial of vacuum extraction
Absence of adequate anesthesia
Inadequate facilities and support staff