Failed Instrumental Delivery Following Combined Clinical and Intrapartum Sonographic Assessment of the Fetal Head Position vs Conventional Labor Management: Propensity Score Matched Analysis of Prospective Data From an International Dataset

Introduction Instrumental delivery is commonly performed during the second stage of labor with the aim to expedite delivery in conditions of second stage dystocia or non-reassuring fetal condition and is estimated to account for approximately 3-15% of all births. The appropriate use of vacuum extractor and forceps is crucial to avoid unnecessary and potentially challenging second-stage caesarean sections, however a 4-6% failure rate has been reported following attempted instrumental delivery. When this occurs, caesarean section is required to deliver the fetus. However, caesarean section following failed instrumental delivery is acknowledged to be associated with an increased incidence of maternal and fetal complications including neonatal trauma, skull fracture, intracranial haemorrhage, neonatal acidaemia and transfer to NICU as well as postpartum haemorrhage, surgical injury and third- or fourth-degree perineal tears compared to a successful vacuum delivery.

The decision to expedite delivery by means of caesarean section during the second stage of labor has also been associated with maternal complications including uterine incision extension, incidental cystotomy and with an increased frequency of fetal injury. Therefore, the decision as to which intervention to perform is crucial to optimize the maternal and neonatal outcomes when an obstetric intervention is indicated during the second stage of labor.

Fetal head position and station are among the major determinants of successful instrumental delivery. Instrumental delivery is recommended following the ascertainment of the fetal head position when the station is below the level of the maternal ischial spines (level 0). A secondary analysis of a randomized controlled trial conducted on a group of women submitted to instrumental vaginal delivery has shown that the risk of failed vacuum extraction is increased in the event of inaccurate placement of the instrument, which is favoured by a high fetal station and a non-occiput anterior position - i.e. a malposition - of the fetal head. Fetal head position and station have been traditionally assessed by digital examination. However, such evaluation has been shown to yield limited accuracy and poor reproducibility. Akmal et al. compared the accuracy of vaginal examination versus transabdominal sonography in the ascertainment of the head position prior to instrumental delivery showing an overall 27% rate of misdiagnosis at vaginal examination, which was mainly accounted by cases in occiput posterior position and station at the level of the ischial spines. Wong et al conduced a randomized controlled trial (RCT) on 50 patients undergoing instrumental delivery by means of vacuum extractor due to second stage dystocia by measuring distance between the centre of the cignon and the flexion point between the cases submitted (n=25) and those not submitted (n=25) to intrapartum US in addition to the vaginal examination. The mean distance was lower in the patients submitted to intrapartum sonography in addition to clinical examination compared to those who had only vaginal examination (2.1+1.3 vs 2.8+1.0 cm, p<0.05).

Conversely, intrapartum sonography has been demonstrated to be a reliable tool for the assessment of the fetal head station and position, particularly in the second stage of labor. The transabdominal approach is considered the gold standard method for the diagnosis of the occiput position, while the transperineal approach is commonly used for the objective measurement of the fetal station. The US parameters have also been shown to be more accurate than clinical examination in predicting the outcome of instrumental vaginal delivery. On this basis, the International Guidelines endorse the use of intrapartum sonography in conditions of abnormal progression of the first or of the second stage of labor and in the event of uncertainty of the occiput position following vaginal examination prior to perform an instrumental delivery.

Hypothesis and Justification for the study