To assess whether ultrasound methods can predict outcome of operative vaginal deliveries in nulliparous women at term with singleton pregnancies and prolonged second stage of labor.

To compare different ultrasound assessments Compare digital assessments and ultrasound findings. Investigate if movement of the fetal head during active pushing is a predictive factor

Null hypotheses:

• Ultrasound measurements cannot predict outcomes of operative vaginal delivery.
• Ultrasound is not better than digital examination in predicting delivery outcome.
• Movement of fetal head with active pushing is not a predictive factor.

The physician responsible for the labor will perform digital examinations (fetal station and position and cervical dilatation). Fetal station will be related to the ischial spine from -5 to +4 as described by WHO and illustrated in Figure 111. Another obstetrician or midwife blinded to the results from the clinical examination will perform the ultrasound measurements. The physician responsible for the labor will be blinded to the ultrasound measurements.

Ultrasound examinations between contractions

Due to considerations described below, only one recording/acquisition will be performed of each of the following:

A) Head position B) Head-perineum distance C) Midline angle D) Angle of progression E) 3D sagittal transperineal acquisition In addition single scans are performed during active pushing as described under B) and D) (and a 3D sagittal scan when possible).

A) Head position Position will be assessed in 2D with a transabdominal scan as described by Akmal4 and 3D in a transperineal scan as described by Ghi et al22. Fetal head position will be recorded as a clock dividing the circle in 24 divisions

Positions ≥02.30 and ≤03.30 hours should be recorded as left occiput transverse and positions ≥08.30 and ≤09.30 as right occiput transverse. Positions >03.30 and <08.30 should be recorded as occiput posterior and positions >09.30 and <02.30 as occiput anterior.

Head-perineum distance will be assessed with transperineal ultrasound. The women will be examined lying flat (or almost flat) in bed with flexed hips and knees position. The bladder should be emptied immediately before the ultrasound examination.

Head-perineum distance will be measured as the shortest distance between the outer bony limit of the fetal skull and the perineum with a transabdominal transducer placed transperineally between the labia in a transverse view (posterior fourchette - posterior commissure of the labia minor)

Midline angle will be measured as described by Ghi22. In a transverse transperineal scan the angle between the midline of the fetal head and a sagittal line through the maternal pelvis will be measured. This recording will also be performed in a transverse scan.

Angle of progression will be measured as described by Barbera and Kalache as the angle between the long axis of the symphysis pubis and the tangent of the skull in a transperineal sagittal scan.

The following outcome variables will be recorded upon delivery Main outcome

• Time from start of vacuum assisted traction to delivery (the entire body is delivered) Secondary outcomes
• Number of contraction before delivery
• Number of cup detachments
• Successful/failed vaginal operative delivery
• Forceps applied/not applied
• Perineal tears
• APGAR score of newborn.
• Arterial umbilical cord blood pH and BD (base deficit) values.
• Position at delivery

Statistics:

The time interval between start of operative vaginal delivery and delivery will be evaluated for the fetal head-perineum distance and angle of progression using survival analyses (Kaplan-Meier analyses and Cox regression analyses).

Categorical variables will be analyzed using Chi-square test and Fischer exact test, and continuous variable will be analyzed using T-test or Mann-Whitney U-test.

The predictive values for a successful operative vaginal delivery will be evaluated using receiver-operating characteristics (ROC) curves. The area under the curve (AUC, - 95% CI) is considered to have discriminatory potential if the lower limit of the CI exceeded 0.5.

Ultrasound measurements, digital assessment of station and dilatation, induction of labor, maternal age, gestational age and birth weight will be analyzed in logistic regression analyses with vaginal delivery vs. cesarean section as dependent variable.

Inclusion Criteria:

• prolonged second stage of labour

Exclusion Criteria:

• Suspected asphyxia before the start of vacuum extraction

Nulliparous women with a singleton live fetus in cephalic presentation at term (≥ 37 weeks) with prolonged active second stage and a clinical decision to expedite delivery by vacuum due to poor progression