Procedure for vacuum-assisted vaginal birth

INTRODUCTION — The decision to use an instrument to assist birth balances the maternal, fetal, and neonatal impact of the procedure against the alternative options of cesarean birth or expectant management. The technique for vacuum-assisted vaginal birth will be reviewed here. An overview of issues related to assisted vaginal birth, including choice of vacuum versus forceps, classification, prerequisites, patient preparation, complications, and outcome, is available separately. (See "Assisted (operative) vaginal birth".)

PREVALENCE — Vacuum-assisted vaginal birth accounts for over 80 percent of assisted vaginal births, 3.6 percent of all vaginal births, and 2.5 percent of all births in the United States [1].

INDICATIONS AND CONTRAINDICATIONS

Indications — An assisted vaginal birth (vacuum or forceps) should only be attempted when a specific obstetric indication is present [2,3]. The three major categories of indication are:

●Prolonged second stage of labor

●Nonreassuring fetal status

●Shortening the second stage for maternal benefit

These indications are discussed in more detail separately. (See "Assisted (operative) vaginal birth", section on 'Indications'.)

Contraindications

●Pregnancy <34 weeks – Historically, experts have recommended avoiding use of vacuum devices before 34 weeks of gestation due to a perceived increased risk of birth injuries in preterm newborns. A registry review of over 40,000 preterm births in Sweden found that 3.3 percent of births <34 weeks were assisted by vacuum despite this recommendation and the frequency of intracranial hemorrhage in births <34 weeks was higher in vacuum-assisted than intrapartum cesarean births (72/1000 versus 59/1000) [4]. While these data were gathered retrospectively and confounded by indication, avoiding vacuum extraction before 34 weeks is a prudent approach.

●Fetal scalp trauma – Prior scalp sampling or multiple attempts at fetal scalp electrode placement are relative contraindications to use of vacuum because scalp trauma from these procedures theoretically may increase the risk of cephalohematoma or external bleeding from the scalp wound.

●Other – The following contraindications apply to both vacuum- and forceps-assisted births:

•Suspected fetal-pelvic disproportion

•Selected fetal and maternal disorders, such as known fetal demineralization diseases (eg, osteogenesis imperfecta), maternal Ehlers-Danlos syndrome [5,6], and fetal bleeding diatheses (eg, thrombocytopenia or hemophilia [7]) are generally accepted contraindications to any assisted vaginal birth because of perceived increased risks of fetal or maternal trauma, but both the absolute and relative risks are poorly defined.

INSTRUMENTATION — Instrumentation consists of a vacuum pump, a cup to attach to the fetal scalp, and a handle attached to the cup by a rigid or nonrigid stem (picture 1).

Vacuum pump — Suction can be generated manually or with an electrical suction device.

The Kiwi complete vacuum delivery system (picture 2), the MityOne one-piece vacuum-assisted delivery system (picture 3), and the Mystic II vacuum-assisted delivery system all come with a pump integrated into the handle of the device, so the operator controls the vacuum without need for an assistant. Some vacuum devices also come with a traction force indicator, allowing the operator to compare tactile impression with an objective measure of force.

Cup — Vacuum cups may be soft (pliable) or rigid and the shape may be bell- or "M"-shaped (picture 4). Rigid cups were initially made of metal, but these have been almost completely replaced by rigid plastic, polyurethane, or polyethylene cups. Soft cups are made of plastic, silicone, rubber, or polyethylene. Sizes vary somewhat by manufacturer; any standard cup size may be used for any late preterm or term fetus meeting criteria for vacuum-assisted birth. (See 'Contraindications' above.)

The optimum type of cup to use for each clinical scenario has not been determined as few randomized trials or comparative studies have been performed.

Soft versus rigid — The performance characteristics of soft versus rigid cups have been examined in numerous trials. A meta-analysis of data from nine trials including 1375 pregnant patients reported the following findings [8]:

●Soft cups resulted in more failures to achieve vaginal birth than rigid cups (failure rate 14.8 versus 9.5 percent, odds ratio [OR] 1.65, 95% CI 1.19-2.29). Most failures were due to cup detachment.

●Soft cups resulted in less scalp injury than rigid cups (13 versus 24 percent, OR 0.45, 95% CI 0.15-0.60).

●The rate of maternal injury was similar for both cups.

●Rigid cups tended to be more suitable for occiput posterior, occiput transverse, and difficult occiput anterior position deliveries, given their ability to stay attached despite strong traction.

●Soft cups appeared to be more appropriate for uncomplicated occiput anterior extractions where less traction is needed and thus the excess risk of scalp injury could be avoided.

Differences in performance between soft versus rigid cups are likely related to cup shape: soft cups are usually bell-shaped while rigid cups tend to be mushroom-shaped (picture 4).

Bell versus mushroom shape — Conclusive studies comparing the clinical performance characteristics of bell-shaped cups versus the Malmström mushroom or "M"-style cups have not been performed. In terms of pure traction force, laboratory studies have demonstrated that bell-shaped cups generate significantly less traction force than M-style cups [9].

This difference in traction between bell-shaped and "M"-style cups may be the result of the interaction between the cup's edges and the scalp chignon that is formed. When bell-shaped cups draw the chignon into the cup, the available vacuum space is reduced, leading to a reduction in cup adhesiveness at the edges, which allows leakage of air and eventual detachments. By comparison, when the mushroom-shaped M-style cups draw the chignon into the cup, the edges interlock with the base of the chignon, thereby creating a mechanical attachment that seems to compensate for the loss of available vacuum space. These theories have been supported by small clinical trials, although none have definitively answered the question of which cup is superior [10,11].

Cup choice for occiput posterior and deflexed heads — A mushroom cup with a nonfixed traction cord is needed for delivery from the occiput posterior position (table 1) because the flexion point of the head is deeper and more posterior in the vagina than with occiput anterior position. A mushroom-shaped cup and a nonfixed traction cord facilitate deep placement in the posterior vagina, whereas the greater height and relatively rigid stem of most bell-shaped cups impede accurate placement in this setting.

A mushroom-shaped cup with a nonfixed traction cord is also useful for deflexed heads since the cup needs to be placed deep in the vagina. One such device has a unique combined handle/vacuum pump (Kiwi OmniCup (picture 2)).

When considering a vacuum-assisted birth with the Kiwi OmniCup in a patient whose fetus is in the occiput posterior position, clinicians should take into account data suggesting occiput posterior position as independent risk factor for both neonatal subgaleal hemorrhage and maternal anal sphincter injury [12].

TECHNIQUE — The clinician must know the indications and contraindications to vacuum-assisted birth and have expertise in proven techniques (see 'Indications and contraindications' above and "Assisted (operative) vaginal birth", section on 'Prerequisites').

Checklists — Checklists can be helpful as cognitive aids for ensuring safety. Checklists have been published by the Society for Maternal Fetal Medicine (SMFM) [13], Royal College of Obstetricians and Gynaecologists [14], and the Society of Obstetricians and Gynaecologists of Canada [15], among others, and can be modified to fit local requirements.

Preparation

●Empty the bladder (via voiding or catheterization).

●Place the patient in the dorsal lithotomy position.

●Confirm preprocedure assessment of fetal position, station, molding, caput, and asynclitism and document findings. Ultrasound examination can be useful if digital examination is uncertain.

We do not administer antibiotic prophylaxis as no benefit has been established, although data are sparse [16].

Anesthesia — Adequate anesthesia can be obtained with a neuraxial technique or possibly with a local block. Vacuum-assisted birth can be attempted without anesthesia if necessary in an emergency, but the patient will likely experience significant discomfort.

Determine the flexion point — The center of the vacuum cup should be directly over the flexion point, which is where outward traction rotates the head toward a midline position, flexes the neck, and pulls the wide mentovertical occipital diameter along the curve of the birth canal. In the normally molded fetal head, the flexion point is in the midline, over the sagittal suture, approximately 6 cm from the anterior fontanelle and 3 cm from the posterior fontanelle (figure 1). Failure to place the cup over the flexion point can impede delivery, rather than assist it.  

Since most of the commonly used vacuum cups have a diameter between 50 and 70 mm, when the center of the cup is placed over the flexion point, the edges of the cup should be approximately 3 cm from the anterior fontanelle and at the edge of the posterior fontanelle. The anterior fontanelle is the reference point for checking the application because access to the posterior fontanelle is partially blocked once the extractor cup is in place.

Insert and place the cup — The practitioner spreads the labia and inserts the bell-shaped cup by compressing and placing it into the vagina while angling posteriorly. If an M-style or rigid cup is used, the device is flexed at the base of the shaft and inserted sideways into the vagina while angling posteriorly.

When the cup contacts the fetal head, the center of the cup is placed over the flexion point and symmetrically across the sagittal suture (figure 1). The entire 360° circumference of the cup must be digitally inspected to ensure that no vaginal, cervical, or vulvar tissues are trapped between the cup and the fetal surface, and that the cup does not cover either fontanelle.

After correct placement is confirmed, vacuum pressure should be raised to 100 to 150 mmHg to maintain the cup's position. The edges of the cup should again be swept with a finger to insure that no maternal tissues are entrapped. The vacuum cup is now properly in place and the higher suction pressures required for traction can be administered.

Rapidly apply suction — As soon as a contraction starts and the mother begins pushing, the negative pressure is rapidly raised to 500 to 600 mmHg (green zone on the vacuum indicator gauge). Although a slow, stepwise increase in suction over 8 to 10 minutes was recommended in the past, randomized trials have demonstrated that rapid application of negative pressure over 1 to 2 minutes reduced the duration of the procedure without compromising effectiveness or safety [17].

Although vacuum suction pressures of 500 to a maximum of 600 mmHg have been recommended during traction, pressures in excess of 450 mmHg are rarely necessary (green zone) (picture 5) [18,19]. Lower suction pressures increase the risk of cup "pop-offs," whereas pressures greater than 600 mmHg increase the risks of fetal scalp trauma and cerebral, cranial and scalp hemorrhage. The notion that the vacuum is "designed to pop-off before damage occurs" is erroneous and should not be considered a safety mechanism. Note: Suction pressure (ie, negative pressure) is measured in various units: 0.8 kg/cm² of atmospheric pressure = 600 mmHg = 23.6 inches of Hg = 11.6 lb/in².

Exert traction during contractions — Traction is applied gradually as the contraction builds and maintained for the duration of the contraction, but only in coordination with the mother's pushing (the number of pulls depends on the number of pushes; there is one "set of pulls" per contraction). Both hands are employed and work in tandem. The fingertips of the dominant hand pull the device's crossbar, while the nondominant hand monitors the progress of descent and prevents cup detachment by placing counter pressure with the thumb [20]. During the extraction, the stem of the device is kept perpendicular to the plane of the cup to maintain the seal with the fetal head. The device is more likely to detach if angular traction is applied or with pendulum or rocking motions. Jerking the device will lead to unnecessary pop-offs.

Traction is applied along the axis of the pelvic curve to guide the fetal vertex, led by the flexion point, through the birth canal. Initially, the angle of traction is downward (toward the floor); the higher the beginning station, the steeper the angle of downward traction required. The axis of traction is then extended upwards to a 45 degree angle to the floor as the head emerges from the pelvis and crowns. The handle of the device is allowed to passively turn as the head auto-rotates through its descent. The handle should never be actively twisted to rotate the head as this dangerous maneuver can cause "cookie cutter" injuries to the fetal scalp [21].

Traction is gradually discontinued as the contraction ends or the mother stops pushing. Descent should occur with each application of traction, beginning with the first.

How much traction? — It is clinically reasonable and practical to rely solely on negative pressure (measured in mmHg), which is displayed on all commercially available devices, as a proxy for traction force.

The absolute "safe" traction force for vacuum extraction is unknown. In 1962, one group determined a total traction force of 17.6 kilogram-force (kgf) (172.6 Newton [N]) was typically necessary to affect delivery [22]. Other authors subsequently determined the traction force to be lower, approximately 12 kgf (117.7 N) in multiparous patients [23-25]. An observational study of 560 vacuum-assisted births using an OmniCup vacuum device with a traction force indicator found that 86 percent of extractions occurred with ≤11.5 kgf (112.8 N) traction and 14 percent with >11.5 kgf (112.8 N) traction [26]. One study demonstrated a threefold increase in neonatal intensive care unit admissions when higher traction forces were employed during the first three pulls (>221 N minutes, which is the sum force N during each pull multiplied by its duration in minutes), although these data were too limited to allow generalized clinical recommendations [27].

In the absence of a traction gauge, traction force (measured in kgf or N) can be calculated based on negative pressure, cup size, and altitude. Cup sizes vary among different manufacturers' devices and cup size affects the overall traction force applied since force = (area under the cup)X(negative pressure). Therefore, as cup size increases, the total force applied will rise even with the same amount of negative pressure [28]. As an example, with 600 mmHg of negative pressure, a 50 mm cup will generate 15.7 kgf of traction force while a 60 mm cup will generate 22.6 kgf of traction force. Whether the greater traction forces associated with larger cup sizes are associated with higher vaginal birth rates or more fetal morbidity is unclear.

Maintaining versus releasing suction — Between contractions, suction pressure can be fully maintained or reduced to <200 mmHg; it is well established that fetal morbidity is similar for both approaches [29].

When the head is delivered, the suction is released, the cup is removed, and the remainder of the birth proceeds as usual. (See "Labor and delivery: Management of the normal first stage".)

Role of episiotomy — Whether routine episiotomy in a vacuum-assisted birth reduces the risk of obstetric anal sphincter injuries (OASIS) has not been established due to the absence of randomized trials and limitations of data from observational studies (eg, study did not distinguish between forceps and vacuum assistance, omission of the type of episiotomy performed, confounding by indication). Observation data suggest a benefit from nonmidline episiotomy.

If the clinician believes an episiotomy is indicated, we favor nonmidline episiotomy in nulliparous patients undergoing vacuum-assisted births.

●In a meta-analysis of 15 observational studies investigating the risk of OASIS in vacuum-assisted births "with" versus "without" use of nonmidline episiotomy (mediolateral or lateral) in a first vaginal birth, nonmidline episiotomy was associated with a reduced risk of OASIS (odds ratio [OR] 0.53, 95% CI 0.37-0.77) [30].

●In a subsequent large retrospective observational study, the incidence of OASIS following vacuum-assisted birth in 130,157 nulliparous patients with and without a mediolateral episiotomy was 2.5 and 14 percent, respectively (adjusted OR 0.14, 95% CI 0.13-0.15); for 29,183 parous patients, the incidence with and without mediolateral episiotomy was 2.0 and 7.5 percent, respectively (adjusted OR 0.23, 95% CI 0.21-0.27) [31].

Rotation from posterior or transverse position — Management of the fetus in occiput posterior or transverse position may involve expectant management, manual rotation, assisted vaginal birth, or cesarean birth. (See "Occiput posterior position", section on 'Management' and "Occiput transverse position", section on 'Approach to patients with transverse arrest'.) For a vacuum-assisted birth:

●Occiput posterior – A vacuum cup designed for the posteriorly positioned occiput should be used (table 1) without attempts at rotation. (See 'Cup choice for occiput posterior and deflexed heads' above.)

●Occiput transverse – A mushroom-shaped cup with a nonfixed traction cord should be used for rotational vacuum deliveries. The handle of the device is allowed to passively rotate as the head auto-rotates from transverse or other off-midline positions to a direct anterior or posterior position as it descends. The handle should never be actively twisted to rotate the head as this dangerous maneuver can cause "cookie cutter" injuries to the fetal scalp [21]. (See "Occiput transverse position", section on 'Forceps rotation'.)

PROCEDURE DURATION — The maximum time to safely complete a vacuum-assisted birth and the number of acceptable "pop-offs" are unknown. A maximum of two to three cup detachments, three sets of pulls for the descent phase, three sets of pulls for the outlet extraction phase, and/or a maximum total vacuum application time of 15 to 30 minutes are often recommended, although most authors advise lower application time limits [25,32-34]. The information for use (IFU) for the Kiwi vacuum system specifically recommends considering abandoning the procedure after two cup detachments (pop-offs) [35].

These recommendations are mostly based upon common sense and experience, keeping in mind that longer procedures or more cup detachments may be surrogate markers for more difficult extractions. However, they are supported by at least two studies assessing the risks associated with cup detachments and duration of the procedure. In both of these studies, the majority of successful vacuum-assisted births was achieved within the duration and detachment parameters recommended, and the risks associated with multiple "pop-offs" (more than three) and/or prolonged procedures (more than 15 to 30 minutes) appear to justify avoiding this type of use in most circumstances, even though the absolute risk of an adverse outcome is low.

●One of these studies described the findings from a secondary analysis of a multicenter observational cohort of 3594 patients who underwent an attempted vacuum-assisted birth and provides insight into the absolute neonatal risks associated with cup detachment and procedure duration [36]. In this study:

•Approximately 60 percent of patients had 0 detachments, 22 percent had 1 detachment, 12 percent had 2 detachments, and 6 percent had ≥3 detachments. The composite adverse neonatal outcome for patients with 0, 1, 2, and ≥3 detachments was 2.4, 3.6, 4.6, and 4.8 percent, respectively. Composite neonatal outcome was defined as any of the following: brachial plexus injury, facial nerve palsy, clavicular fracture, skull fracture, other skeletal fracture, skin laceration, intracranial hemorrhage (including subgaleal hemorrhage [SGH]), seizure requiring treatment, or neonatal death. Skin laceration was the most common individual adverse outcome.

•Procedure duration was defined as the time from first application of the vacuum to either time of vaginal birth or time of decision to convert to a cesarean. Composite adverse neonatal outcomes for durations of 0 to 2, 3 to 5, 6 to 8, 9 to 11, and ≥12 minutes were 1.1, 2.8, 3.6, 3.7, and 5.0 percent, respectively. In multivariate analysis, increasing procedure duration was more predictive of adverse neonatal outcome than an increasing number of detachments.

●The second study of 700 vacuum-assisted births compared 350 neonates with subgaleal hemorrhage (SGH) with 350 matched controls without SGH to evaluate factors associated with SGH formation [37].

•Risk of SGH more than doubled with cup detachment: for each cup detachment the OR was 2.38, and three or more cup detachments were independently associated with SGH formation.

•Procedure duration ≥15 minutes (defined as the time of first application of the vacuum to vaginal birth) was independently associated with SGH with an adjusted odds ratio of 2.04 for each three minute increase.

DOCUMENTATION — Documentation of a vacuum-assisted birth should include all of the following:

●The indication for the procedure

●Fetal status (station [on a scale of 3 or 5], position, estimated fetal weight, interpretation of the fetal heart rate tracing)

●A description of the discussion with the patient

●A description of the procedure itself, including:

•Type of anesthesia

•Type of vacuum cup

•Maximum negative pressure achieved

•Total time of negative pressure and whether the negative pressure was reduced between contractions

•Number of pulls and contractions

•Number of detachments

•Description of progress with each pull

•Whether an episiotomy was performed and type

•Type and occurrence of lacerations

It is also worth documenting that the prerequisites for vacuum-assisted birth were met: cervix fully dilated, maternal bladder empty, and absence of known fetal contraindications (eg, the gestational age was ≥34 weeks, no known fetal bleeding diathesis, etc).

FAILED PROCEDURES

Causes of failure — After an unsuccessful procedure, it is important to conduct a step-by-step review of events to identify areas for future modification of technique or decision-making.

The reasons for failure are multifactorial, and include:

●Fetopelvic disproportion (eg, non-occipitoanterior position, macrosomia, uterine constriction ring).

●Incorrect technique – Pulling the stem too quickly or when maternal expulsive efforts are weak will lead to cup detachments. Moreover, upwards traction before the head is crowning will tend to disrupt the vacuum seal and lead to pop-offs. Incorrect cup size also increases the risk of failure.

●Paramedian or deflexing applications – It is essential to identify the flexion point and focus the vacuum traction to leading the mentovertical diameter through the pelvis. Off-midline applications or deflexing applications will pull less favorable diameters of the fetal head through the pelvis and inhibit, rather than assist, delivery [38,39].

●Large caput succedaneum – An increase in fetal scalp edema lets more of the scalp to be drawn into the cup, which reduces the available vacuum area, and, in turn, lessens total traction. The effect is more pronounced with bell-shaped cups than in M-style cups [9], and with soft cups compared with rigid cups [8].

Risk factors — In a retrospective case-control study of 306 failed and 618 successful vacuum-assisted births [40]:

Predictors of a failed procedure included:

●Estimated fetal weight ≥3750 g as compared with <3250 g (odds ratio [OR] 5.7)

●Epidural analgesia (OR 3.0)

●Occiput posterior position (OR 2.6)

●Failure to progress as the indication (OR 1.7)

●Labor augmentation (OR 1.4)

●Increasing gestational age (OR 1.2 per week)

Predictors of a successful procedure included:

●At least one previous vaginal birth (OR 0.32)

●Lower station of the fetal head at start of the procedure (OR 0.31 per station more descended)

●Taller maternal height

Potential tools for predicting failure — Using ultrasound to measure the distance from the presenting part of the fetal head to the maternal perineum, and then predicting chances of procedure success based on this number, is an investigational tool. Head-to-perineum distances >35 mm in one study and >40 mm in another study, which correlated with +2 station, were associated with higher failure rates and more difficult vacuum-assisted births [41,42].

Another investigational technique uses a combination of the ultrasound-measured angle of progression with pushing (AoP) and fetal head circumference to predict complicated versus uncomplicated vacuum- or forceps-assisted vaginal births. In this model, a combination of an AoP with pushing of less than 115° and a fetal head circumference greater than 345 mm predicted 87 percent of complicated assisted vaginal births [43].

Management after a failed procedure — We suggest prompt cesarean birth after an unsuccessful vacuum-assisted procedure. Failure of an attempted vacuum-assisted birth increases the likelihood of neonatal morbidity [44]; the subsequent use of sequential forceps in this setting has been associated with an increased risk of neonatal intracranial hemorrhage [45] and is rarely indicated.

TIPS FOR REDUCING THE RISK OF COMPLICATIONS — All assisted vaginal births carry some risk of potentially serious complications. No studies have clearly demonstrated a benefit of one type of vacuum cup over another for these complications. Similarly, no threshold for duration of vacuum application or maximum number of pop-offs has been proven to prevent serious complications.

The following tips are useful for reducing the risk of complications:

●Comply with professional society guidelines – In a retrospective study, compliance with the Royal Australian and New Zealand College of Obstetricians and Gynaecologists' guidance on instrumental vaginal birth was associated with lower rates of subgaleal hemorrhage (0 versus 11 percent) and major birth trauma (3 versus 22 percent) compared with noncompliance [46]. The main deviation from compliance was pulling more than three times.

●Confirm correct cup placement — A successful vacuum-assisted birth requires placement of the cup over the flexion point. Misalignment of the cup relative to the flexion point leads to cranial deflexion or asymmetry as traction is applied, which impedes, rather than assists, descent because a larger cranial diameter is presented to the birth canal. Paramedian application is also associated with a higher rate of neonatal scalp trauma [26].

If the cup is dislodged, examine the scalp for any injury before reapplying the cup. The cup should not be placed over an injured scalp.

●Avoid entrapping vaginal soft tissues — Entrapment of maternal tissues between the cup and the fetal head will cause vaginal and/or vulvar lacerations, which can be difficult to repair and cause unnecessary maternal blood loss and discomfort.

●Know when to abandon the procedure — As with any obstetric intervention, practitioners must be willing and able to abandon the procedure and proceed to cesarean birth promptly when the vaginal birth is not progressing normally [47]. Although there is often a tendency to try to complete a vaginal birth despite failed progress and/or multiple "pop-offs," prudence dictates moving to an abdominal birth when the fetus is not readily delivered with vacuum assistance. An indicated vacuum-assisted vaginal birth that could not be completed is unlikely to progress to a spontaneous vaginal birth with a little more time, and delay may increase the risk of neonatal or maternal morbidity. (See 'Procedure duration' above.)

COMPLICATIONS AND OUTCOME — The complications and outcomes of vacuum-assisted birth are discussed separately. (See "Assisted (operative) vaginal birth", section on 'Complications'.)

SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Delivery".)

SUMMARY AND RECOMMENDATIONS

●Indications and contraindications – The three major categories of indication are prolonged second stage of labor, nonreassuring fetal status, and shortening the second stage for maternal benefit. In addition to the general contraindications to assisted vaginal birth, contraindications specific to vacuum extraction include gestational age less than 34 weeks and previous scalp sampling. (See 'Indications and contraindications' above.)

●Choice of cup – There is insufficient evidence upon which to base a recommendation for use of a particular type of vacuum cup (table 1) for all circumstances when a vacuum-assisted birth is attempted. A successful vacuum extraction is most likely to be achieved by accurate cup application, appropriate traction technique, a favorable flexed fetal cranial position and low station at the time of application, use of the most appropriate cup design, and absence of fetopelvic disproportion. (See 'Cup' above and 'Technique' above.)

●Procedure

•Cup placement – The cup should be applied at the flexion point and the edges swept with a finger to insure that no maternal tissues are entrapped. In the normally molded fetal head, the flexion point is in the midline, over the sagittal suture, approximately 6 cm from the anterior fontanelle and 3 cm from the posterior fontanelle (figure 1). (See 'Determine the flexion point' above.)

•Vacuum pressure – Rapid application to the maximum suction pressure of 600 mmHg is acceptable, although pressures in excess of 450 mmHg are rarely necessary. Slow, stepwise application of suction does not improve safety or efficacy. Between contractions, suction pressure can be fully maintained or reduced to <200 mmHg. (See 'Rapidly apply suction' above and 'Exert traction during contractions' above.)

•Traction – Apply gentle traction along the axis of the pelvic curve (ie, down then up) in concert with maternal pushing. The scalp can be damaged if the handle is actively twisted to rotate the head. A mnemonic is provided in the table (table 2) to assist in remembering the steps in vacuum extraction. (See 'Exert traction during contractions' above.)

•Number of attempts – We suggest limiting vacuum-assisted procedures to three contractions for the descent phase, three contractions for the outlet extraction phase, 2 to 3 "pop-offs," and a total time of 15 to 30 minutes (Grade 2C). (See 'Procedure duration' above.)

●Failed procedures – Failure of an attempted vacuum-assisted vaginal birth increases the likelihood of neonatal morbidity; the subsequent use of sequential forceps in this setting is rarely indicated. We suggest prompt cesarean birth after an unsuccessful vacuum-assisted procedure. (See 'Failed procedures' above and 'Tips for reducing the risk of complications' above.)