The use of forceps or vacuum extraction (VE) to facilitate delivery of the baby’s head during the second stage of labor
Forceps: Figure 4.4.1
Vacuum extractor: Figure 4.4.2
Figure 4.4.3 shows the four parent types of maternal pelvis (Caldwell-Moloy classification).
Table 4.4.1 lists some important diameters of the maternal pelvis.
Performing clinical pelvimetry
Can only be adequately evaluated before engagement of the fetal head because once the head occupies the midcavity, the posterior inlet cannot be accessed
Anteroposterior (AP) diameter
Diagonal conjugate = distance from the undersurface of the symphysis pubis to the sacral promontory
Obstetric conjugate (the narrowest AP diameter of the inlet) is estimated by subtracting 2 cm from the diagonal conjugate.
Transverse diameter of the inlet cannot be measured clinically.
The shape and the extent of the circumference of the inlet—sweep fingers laterally along the pelvic brim. If greater than two-thirds of the brim and/or posterior portions of the brim can be felt = contracted inlet.
Shape of the sacrum (curved or straight)
Width of the sacrosciatic notch
Prominence of, and distance between, the ischial spines
A contracted midpelvis characteristically shows a flattened forward-projecting sacrum, prominent ischial spines with a narrowed interspinous distance, and a shortened sacrospinous ligament that is less than two fingerbreadths long.
Table 4.4.1 Diameters of the Female Pelvis
Region of the Pelvis
Brim (inlet): AP
Midcavity (midpelvis: AP and transverse)
Cavity: AP and transverse
Distance between the ischial tuberosities (normally ˜10 cm)
Palpate coccyx: Normally mobile/not protruding into the pelvic cavity
Subpubic angle (normal > 90 degrees)
Retropubic angle (flattened in a platypelloid pelvis and sharply angulated in an android pelvis)
Convergence or divergence of the pelvic sidewalls (Figure 4.4.3).
During labor, for a complete picture of the situation clinical pelvimetry should be combined with fetal parameters such as the following:
Head position, molding, caput succedaneum, asynclitism, maternal and fetal soft-tissue edema, presence of meconium, and descent of the head
Fetal heart rate response to contractions and/or pushing.
Some important definitions are shown in Table 4.4.2:
American College of Obstetricians and Gynecologists (ACOG) (1) Classification of Station: Level of the leading bony point of the fetal head in centimeters at or below the level of the maternal ischial spines (-0 cm to +5 cm)
Applied fetal cephalic anatomy (2)
Figure 4.4.4 shows important landmarks.
Figure 4.4.5 presents relevant diameters of the fetal skull.
Table 4.4.3 shows the diameters and circumferences of the fetal head that will need to negotiate the maternal pelvis in specific fetal head positions.
Table 4.4.2 Important Definitions Useful in Clinical Pelvimetry
The widest diameter of the fetal presenting part (in most cases the biparietal diameter of the fetal head) is at or below the plane of the maternal pelvic inlet. The best method of determining this is a combination of abdominal and pelvic examination.
The relationship between the leading bony part of the fetal presenting part (usually the cranium) and the maternal ischial spines. Usually, but not always, the fetal biparietal diameter is engaged when the leading portion of the skull is felt at 0 station. The most common source of error in assessing station is caused by the cephalopelvic disproportion. In circumstances where severe caput succedaneum precludes accurate diagnosis of the station, operative vaginal delivery should not be contemplated.
The relationship of the fetal head to the fetal spine. The head may be flexed, extended, or in a neutral attitude (“military attitude”).
The relationship of the denominator of the fetal presenting part to the maternal pelvis. In a cephalic presentation, the denominator is the occiput, while in a breech presentation it is the sacrum. The position is always described in relation to the maternal left and right sides of the pelvis.
The relationship between the leading fetal part and the maternal pelvic inlet. The fetus may have a cephalic, breech, or shoulder presentation.
The relationship between the fetal and maternal longitudinal axes, which may be longitudinal, oblique, or transverse.
The relationship between the anterior and posterior parietal bones and the sagittal suture. When neither of the parietal bones precedes the sagittal suture, the head is synclitic; if the anterior parietal bone precedes the sagittal suture, there is anterior asynclitism; and when the posterior parietal bone precedes the sagittal suture, there is posterior asynclitism.
This term describes the phenomenon of abutment or overlapping of the fetal skull bones caused by excessive pressure on the fetal head. In general the occipital and frontal bones slip under the parietal bones when molding occurs.
Swellings seen on the neonate’s head (2):
Figure 4.4.6 shows types of swelling that may be seen on the neonatal head:
Caput succedaneum: Serous effusion between aponeurosis and periosteum overlying the leading part of the skull. Normal occurrence, from pressure from the cervix. There are varying degrees described subsequently. Usually disappears within hours of birth.
Table 4.4.3 Diameters and Circumferences of the Fetal Head That Will Need to Negotiate the Pelvis With the Head in Specific Positions
Nape of neck to center of bregma
Below chin to center of bregma
Point of chin to above posterior fontanelle
Base of skull to most distant point of vertex
Root of nose to occipital protuberance
Between two parietal eminences
Greatest distance between the two halves of the coronal suture
Subocciptobregmatic × biparietal
Occipital-frontal × biparietal
Deflexed vertex and occipital posterior positions
Mentum-vertical × biparietal
Brow presentation (largest possible diameter)
Caput succedaneum should be differentiated from the following two life-threatening bleeding differential diagnoses:
Cephalohematoma: Collection of blood between periosteum and skull bone, limited by periosteal attachments at the suture lines. Can still be significant. Takes hours to develop and weeks to reabsorb.
Subgaleal (subaponeurotic) hematoma: Potential space between galea aponeurotica (epicranial aponeurosis) and periosteum (pericranium). No boundaries—may extend from the orbital ridges to the nape of the neck with possibility of life-threatening hemorrhage. Presents with diffuse swelling of the head and signs of hypovolemic shock (e.g., pallor, hypotension, tachycardia, and increased respiration rate). The signs may be present at delivery or develop several hours or up to a few days following delivery. The swelling shifts dependently and indents easily on palpation. May be difficult to distinguish from the edema of the scalp, but any hypotension and pallor after an operative vaginal delivery (OVD) should be regarded as potential signs of a subgaleal bleed.
Molding: Molding is an expected deformation of the fetal head that occurs during the delivery process. A small amount of occipitoparietal molding is normal during the second stage, but excessive skull bone molding (particularly parietoparietal molding) is abnormal during the first stage of labor. Molding can be classified in the following way (Figure 4.4.7) (2,3,4,5):
Figure 4.4.7. Degrees of molding of the fetal head. Note that in the bottom panel the membranes underlying the suture have torn, leading to intracranial bleeding.
0+ = suture easily felt between the two bones
1+ = no suture felt, bones easily separated by minimal digital pressure
2+ = overlapping bones, separated with digital pressure
3+ = overlapping bones, cannot be separated with digital pressure
Fetal head position
It is axiomatic that until the exact head position and station are known, OVD should not be attempted.
If necessary, ultrasound can be used to confirm clinical findings, either translabially or transabdominally (6).
Routine assessment before OVD to confirm the lie and identify relationship of fetal back to the uterine midline
If fetal back is not felt, or is palpated laterally, there is a higher likelihood of the baby being in an occipitoposterior or transverse position.
Assess fetal weight.
The average adult hand is ˜10 cm across from thumb to little finger. Thus, each finger is ˜2 cm in width, or one-fifth of the hand. Using the number of fingerbreadths of the fetal head palpated above the maternal symphysis, an estimation on the “number of fifths” of the fetal head can be made.
If greater than two-fifths of the fetal head (two fingerbreadths) is palpable above the pubic symphysis, regardless of the fact that the scalp is felt at the ischial spines, the head should be regarded as unengaged and an operative delivery should be avoided.
Value of the abdominal examination (5)
Prediction of successful OVD has been studied and shown to be better when using abdominal examination criteria (including an assessment of the amount of fetal head palpated above the pubic symphysis) (94%) than by vaginal examination criteria (80%) (p < 0.01).
No longer employed except in low-resource environments where ultrasound and/or magnetic resonance imaging (MRI)
is not available. X-ray pelvimetry may be useful in such environments to rule out pelvic abnormalities.
May be useful to determine fetal size, presentation, position, station, and viability in labor
Figure 4.4.10. A: Schematic representation of ultrasound techniques to identify fetal position and presentation: Transabdominal scan is first obtained to demonstrate spine and occiput (1), and transperineal scan is then performed to identify cerebral midline echo (2). B: Schematic representation of direction of sagittal suture before internal rotation. C: Sonogram corresponding to section plan (1): Wide angle between fetal spine and occiput indicates normal flexion. D: Sonogram corresponding to section plan (2): Following engagement and before internal rotation, cerebral midline echo has an angle of about 45 degrees to the anteroposterior axis of the maternal pelvis. (Reprinted from Bellussi F, Ghi T, Youssef A, et al. The use of intrapartum ultrasound to diagnose malpositions and cephalic malpresentations. Am J Obstet Gynecol. 2017;217(6):633-641.)
May be useful for pelvimetry or for diagnosing fetal anomalies in well-resourced environments
Prerequisites for OVD
Table 4.4.4 presents a mnemonic that is useful in ensuring the best chance of success for an OVD.
Contraindications to OVD (Table 4.4.5)
Recommended in all but emergent circumstances
Table 4.4.4 Prerequisites for an Operative Vaginal Delivery
Favorable head position (OA/OP) <45 degrees from the AP diameter)—unless intended rotational delivery
Fetal assessment (weight and status)
Open Os (completely dilated cervix)
Rule of three’s, Ruptured membranes
Contractions present, Consent (verbal or written)
Engaged head, Empty bladder, Epidural or other form of adequate anesthesia
Prepared for cesarean section, if needed
Prepared for neonatal resuscitation, if needed
Pelvimetry (clinical) adequate
Preoperative note written
Stirrups and lithotomy position with attention paid to pressure points and prevention of hyperflexion at the hips (to avoid femoral nerve apraxia)
AP, anteroposterior; OA, occipitoanterior.
Table 4.4.5 Contraindication to Any Operative Vaginal Delivery
A noncooperative patient, or one who refuses operative vaginal delivery
A fetus with a known bone demineralization condition (e.g., osteogenesis imperfecta) or bleeding diathesis (e.g., alloimmune thrombocytopenia, hemophilia)
Incomplete cervical dilation
An unengaged fetal head
An unknown/uncertain position of the fetal head
Inability to achieve proper application of the chosen instrument
A prior failed operative vaginal delivery attempt (unless there is an obvious reason for the failure unrelated to cephalopelvic disproportion that is unlikely to occur again, and there is a justification for a second attempt)
A known fetal weight in excess of 4,500 g
Contraindications specifically to vacuum extraction
Fetal prematurity (<34 weeks of gestation)
Fetal scalp trauma
Delivery requiring rotation > 45 degrees or excessive traction
Document indication (Table 4.4.6) and satisfied prerequisites (Table 4.4.4) and expected type (Table 4.4.7) of OVD, as well as the fact that risks and benefits were discussed, assent or informed consent given, and the alternative of a cesarean section offered and declined. Patients should be informed of the risk of intracranial hemorrhage and neonatal intensive care unit (NICU) admission—Towner et al. (7) provide useful population-based information showing a similar risk of intracranial bleeding following OVD versus cesarean birth after prolonged labor. The reader is encouraged to read the Towner et al. paper to become familiar with the increased relative risk of OVD under various circumstances such as failed OVD and sequential use of OVD instruments.
Regional block: Epidural or spinal conduction anesthesia is optimal.
Pudendal block and perineal infiltration if regional is not possible.
General anesthesia may be indicated under individualized circumstances.
Assembling the forceps or vacuum extractor before use
Check that all parts fit together and function well.
Held in a way that simulates the intended application and use
Modified lithotomy position
In special cases (such as cardiac disease), this should be modified with the patient in a Fowler position with their feet on small stools below the level of the hips.