The majority of women have spontaneous vaginal birth but some women need assistance with delivery of the baby in the second stage of labour. Instrumental vaginal delivery is when obstetric forceps or the vacuum extractor is used to assist delivery of the baby. Rates of instrumental vaginal delivery range between 5% to 20% of all births in industrialised countries (Janni 2002; Roberts 2002a). Indications for instrumental vaginal delivery may be for fetal or maternal reasons or for both. Fetal conditions in which instrumental vaginal delivery may be required include fetal heart rate abnormality and malposition of the fetal head such as occipito‐posterior position. Maternal exhaustion and medical conditions in which expulsive effort must be reduced such as cardiac disease may necessitate an assisted second stage. The majority of instrumental vaginal deliveries are conducted in the delivery room but in a small proportion (2% to 5%) a trial of instrumental vaginal delivery is conducted in theatre with preparations made for proceeding to caesarean section (Mola 2002; Murphy 2001b). Although all instrumental vaginal deliveries can be considered a trial, there are clinical factors such as malposition and level of presenting part may influence the operator in deciding on delivery in the room or in theatre. However, there are no agreed criteria for a trial of instrumental vaginal delivery in theatre. The decision to attempt instrumental vaginal delivery in theatre is subjective with wide variation between practitioners and is influenced by the local practical arrangements for conducting a trial in theatre.

Some trials of instrumental vaginal delivery fail to complete delivery by the vaginal route and caesarean section is performed. Failure rates of instrumental vaginal delivery range from 16% to 20% (Cheong 2004; Sau 2004). Failed trial of instrumental vaginal delivery in theatre is associated with a shorter interval between failure of trial and delivery by caesarean section when compared to a failed attempt conducted in the delivery room. Factors that reduce chances of successful trial of instrumental vaginal delivery include nulliparity, malposition of the fetal head, high level of head, deflexed head and reduced pelvic capacity (Al‐Kadri 2003). The skills of the operator affect outcome of trial of instrumental vaginal delivery. An experienced practitioner is likely to have a higher success rate by careful patient selection. A cautious less experienced operator may also have a high success rate through conducting the majority of instrumental vaginal deliveries as trials in theatre. In some settings there are protocols for a trial of instrumental vaginal delivery in theatre such as requiring senior obstetrician approval.

The second stage of labour is defined as the interval between achieving full cervical dilatation and delivery of the baby. The second stage can be subdivided into a passive phase where the woman has no urge to push and an active phase when the woman is bearing down during contractions. During the passive phase the presenting part descends to below the ischial spines and the fetal head tends to rotate to an occipito‐anterior position. The optimal duration of the second stage remains undefined but a mean of 70 minutes, with 79% of women experiencing a second stage of less than two hours has been reported (Janni 2002). There is, however, no evidence that a second stage lasting more than two hours was detrimental to the fetus if adequate monitoring was provided. The duration of the second stage can be affected by use of analgesia especially epidural that may remove urge to push (Liu 2004; Zhang 2001). In women with an epidural, directed pushing should be discouraged until the presenting part descends below the spines. Delaying pushing to more than two hours after full cervical dilatation reduces difficult deliveries (Fraser 2000). The phase of active pushing is accompanied by a decline in fetal pH and there are recommendations for this to be shortened (Roberts 2003). The second stage is generally shorter in multiparous women. Failure to progress in the second stage may be due to malposition of the fetal head, ineffective uterine contractions or feto‐pelvic disproportion. An oxytocin infusion commenced in the second stage to correct ineffective uterine contractions shortens the second stage and reduces instrumental deliveries. It is recommended that the active phase should not exceed an hour in primigravida and should be shorter in multiparous women (Murphy 2001a; Sizer 2000). Increased rates of caesarean section, instrumental vaginal delivery and perineal trauma (third and fourth degree tears) with second stage longer than one hour have been reported (Cheng 2004). Although length of the second stage was not associated with poor neonatal outcome, prolonged second stage is associated with increased maternal morbidity and instrumental vaginal delivery rates. There is no evidence to support arbitrary time limits for the duration of the second stage. Active pushing is associated with fetal acidosis and denervation injury to maternal perineal muscles (Roberts 2002b). Maternal positions in labour that promote descent of the presenting part are helpful in shortening the duration of the second stage.

The decision to perform instrumental vaginal delivery in theatre indicates the operator anticipates a difficult delivery with a higher than average risk of failure. Trial of instrumental vaginal delivery has been considered an acceptable alternative to caesarean section for delay in the second stage due to a potentially difficult mid‐cavity arrest (Lowe 1987). There are reports that suggest increased maternal and neonatal morbidity following failed trial of instrumental vaginal delivery (Murphy 2003; Sadan 2003; Sheiner 2001) and others suggesting that failed trial of instrumental vaginal delivery does not increase maternal and neonatal morbidity (Murphy 2001b; Revah 1997). Immediate caesarean section without trial of instrumental vaginal delivery may increase caesarean section rate by performing caesarean section in women who could have a successful trial of instrumental vaginal delivery. The fetal head is likely to be impacted in second caesarean section at full cervical dilatation especially following failed instrumental delivery. Disengagement of the deeply wedged head during caesarean section can be difficult as well as traumatic (Blickstein 2004). Techniques for disengagement include pushing by an assistant or pulling the baby by the feet and delivery by breech extraction. Caesarean section also has implications for future pregnancies with an increased risk of repeat operation and scar rupture. However, a difficult instrumental vaginal delivery may be associated with increased neonatal morbidity (Johnson 2004; Sheiner 2001). Long‐term effects of instrumental vaginal delivery include perineal pain, incontinence and sexual problems (Burrows 2004; Liebling 2004).

There is variation in threshold for performing trial of instrumental vaginal delivery . Some practitioners feel that if there is a 50% chance of success, trial of instrumental vaginal delivery is justified. When instrumental vaginal delivery is set up as a trial, high failure rates have been reported (Lowe 1987).

We will not assess in this review the merits of the different instruments used for instrumental vaginal delivery which was the subject of a previous review (Johanson 1999). This review concluded that compared to forceps, the vacuum extractor appeared to reduce maternal morbidity. Local practice and operator preference also influence the choice of instrument.