Angle of progression and vaginal delivery

Uncomplicated vaginal birth is the favourable outcome in most pregnancies. If
women fail to progress in the second stage of labor, obstetric intervention is
needed. Obstetric interventions include Cesarean section(15.6%)1, instrumental
delivery using forceps or vacuum extraction(10%)1 and secondary Cesarean
section after failed instrumental delivery or in case of fetal distress.
A study showed that compared to a successful instrumental delivery, the risk of
intracranial haemorrhage increases from 1:860 to 1:334 after failed operative
vaginal delivery followed by Cesarean section2. In addition, second-stage
Cesarean section yields increased maternal risk as well. Since the fetal head
is embedded deeply into the maternal pelvis, it is associated with major
haemorrhage, increased risk of bladder trauma and broad ligament hematomas3,4.
Predicting the chance of vaginal delivery is commonly based on digital vaginal
examination, where progression of cervical dilatation and fetal head descent
are assessed by palpation. However, studies showed that this *golden standard*
is a subjective evaluation with several limitations, such as a high error rate
of determining fetal head position5-10.
Recently, intrapartum transperineal ultrasonography (ITU) has been studied to
predict the mode of delivery. Various ultrasound parameters have been
described, including angle of progression, head-perineum distance,
head-symphysis distance, mid-line angle, pubic arch angle and fetal direction
and rotation. However, the angle of progression (AOP) was found to be the most
reproducible parameter for predicting the mode of delivery11. The AOP is
defined as the angle between a line drawn through the midline of the pubic
symphysis and a line drawn from the inferior edge of the symphysis tangentially
to the fetal skull12.
Kalache et al. found that when the AOP was 120*, the probability of an easy and
successful vacuum extraction or spontaneous vaginal delivery was 90%13.
Furthermore, Sainz et al. showed that if a progression angle is <= 120*, the
foetal head direction is horizontal or head-down and a midline angle is >= 35*
before the vacuum is placed, there is a 85% probability that the delivery will
require more than 4 vacuum pulls14. Ghi et al. found that the AOP was a
significant predictor of duration of the active second stage of labor and that
it might play an important role in predicting the mode of delivery15,16.
Barbera et al. assessed the feasibility and reproducibility of measuring the
AOP by transperineal ultrasound. The study showed good levels of intraobserver
variability (approximately 2.9 degrees), and interobserver error (1.24
degrees), indicating the accuracy of the technique12. In addition, where
digital examination is often misleading by caput succedaneum formation, head
oedema and other pelvic landmarks, these elements will not affect the angle
measured, since it only depends on two bony structures with clear landmarks
(the pubic symphysis and the calvarium) 5,12.
ITU seems to be an objective and reliable tool to predict the mode of delivery.
Based on quantative data, it provides more accurate details about the progress
of labor than the traditional digital examination5-7, 10, leading to timely and
appropriate decision making during the second stage of labor and improved
obstetric outcomes. Although these results are promising, little is known about
the use of AOP in predicting the mode of delivery and the existing data relies
upon a small number of patients included in the studies.
The aim of this study is to establish a predictive model for the probability of
vaginal delivery and successful vacuum extraction at the second stage of labor,
using the AOP by transperineal ultrasound. Therefore, a cut-off point for
predicting successful vaginal delivery and successful vacuum extraction will be
generated. Also the use of GE Healthcare SONO L&D software in measuring the
angle of progression will be studied, as well as the experiences of women in
labor using ITU.

Primary Objective:
To establish a prediction model for the probability of vaginal delivery and
successful vacuum extraction at the second stage of labor, using the angle of
progression by transperineal ultrasound.

Secondary Objective(s):
To evaluate the experiences of women in labor using ITU, using questionnaires.
To evaluate the use of GE Healthcare SONO L&D software in measuring the angle
of progression.

This will be a prospective cohort study of the angle of progression in the
second stage of labor. All women with singleton pregnancies and with a
gestational age >37 weeks with spontaneous or induced onset of labor planning
to deliver at the Obstetric Department of the UMCG will be included. We plan to
include 250 women. Women will be informed about the study during prenatal
visits at our outpatient clinic or before an induction is started at the ward
and asked for consent. Written informed consent, inclusion & exclusion criteria
will be checked for every patient. When the woman reaches the second stage of
labor, an ultrasound examination will be performed every 20 minutes until the
moment of delivery. Women are considered to be in the second stage of labor
when they reach full cervical dilatation and they feel the urge to start
pushing at every contraction. Digital examination as routine care will be used
to define the start of the second stage. The transperineal ultrasound will be
performed by a Voluson P8 ultrasound system (GE Healthcare). The ultrasound
probe, wrapped in a sterile plastic bag, will be gently applied to the labia
majora of the women with the use of sterile coupling gel between probe and
maternal tissues. The angle of progression will be measured automatically by
the built-in SONO L&D software by drawing a line through the midline of the
pubic symphysis and another line from the inferior edge of the symphysis
tangentially to the fetal skull. The angle will be stored blindly in the
equipment and it will not influence clinical management, which will be as
usual. Intraobserver and interobserver variability will be calculated.
Intraobserver variability will be determined by replicating the measurement on
25 patients by the same observer at approximately the same time. To determine
interobserver variability, measuring the angle of progression will be repeated
by a second observer to a subset of 25 randomly selected women. Data collection
will be checked for every 5th patient and stored anonymously. The measured
angles will be analysed and a cut-off for spontaneous vaginal delivery will be
established. Women will be requested to fill in a short questionnaire
containing 5 multiple choice questions regarding their experiences with
transperineal ultrasound during the second stage of labor.

Transperineal ultrasound is not associated with additional risks for the mother
or the fetus or excessive discomfort. The investigation consists of applying a
probe against the labia majora of the parturient during the second stage of
labor. ITU will last not more than a few minutes since the software measures
the angle automatically and it will be repeated every 20 minutes in case of
prolonged second stage. After delivery women are requested to fill out a short
questionnaire containing 5 multiple choice questions. The questionnaire is
given at the moment the child will be checked and weighted, which is usually a
moment of peace in the delivery room. In case a cesarean section is performed,
the questionnaire is given at the maternity ward. Before the patient is
dismissed from the hospital, the questionnaire will be checked.