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50. Ultrasonography in Labor
50.1 Introduction
Digital pelvic examination has been the gold standard of monitoring the progress of labor, even though it is quite uncomfortable for the patient. It is imprecise [1] and subjective in nature. It has high interobserver variations and inaccuracies [2] in the assessment, which is a well-established fact. With the advent of portable machines and availability of ultrasound machines in the labor room, now the concept of sono-obstetrician is evolving rather than just the obstetric sonogram.
Over the last 30 years, intrapartum sonography has gone a long way from the assessment of the fetal heart rate in labor or locating the placenta to assessing the progress of labor. There are now extensive studies available which explore the use of this noninvasive, cheap, and highly informative modality in assessing the progress of labor, identifying the cases of arrest of labor, and timely deciding the mode of delivery.
Basic uses |
1. Identification of fetal heartbeat |
2. Determination of fetal presentation |
3. Estimation of fetal weight |
4. Site of the placenta |
5. Determination of the degree of flexion of the head |
6. Assistance in the birth of second twin |
7. Postpartum bleeding |
Advanced uses |
1. Determination of fetal head position, station, and rotation |
2. Diagnosis of protracted labor |
3. Assessment of intrapartum fetal well-being |
50.2 General Setup During Labor
Examiner: The intrapartum ultrasound should be done by an expert who has the knowledge about the use of ultrasound as well its use in labor. As we still know, that use of ultrasound in labor is not a routine in many centers. Still, in our teaching, there is a great gap between knowing the delivery skills and obstetricians having sonography skills.
Ultrasound probes: It is not really required to have really expensive machines to do the intrapartum sonography. I have used convex abdominal as well as the volume probes. I have not used transvaginal probes for doing intrapartum ultrasound, though there are some studies which are using TVS probes for measuring preinduction cervical length.
50.3 Ultrasound Machine Settings
- 1.
Lowest possible angle of insonation
- 2.
Lower output frequency
- 3.
Highest insonation depth
- 4.
Wide volumetric area with low sound volumes
50.4 Types of Ultrasound Exploration Techniques
- 1.
Transabdominal—The abdominal probe (convex or volume) is kept transversally in the suprapubic region of the maternal abdomen. This is mainly for assessing head rotation.
- 2.
Translabial or transperineal—The abdominal probe is placed inside a rubber glove covered with an ultrasound gel and then placed longitudinally in the medial sagittal position between both labia majora and below the symphysis pubis with right side of the probe facing toward the anus (Fig. 50.1).
Turing the probe transversely will give the cervix and head.
Transperineal sagittal application of the 2D transducer (a) gives views of the maternal symphysis pubis and the upper part of the cervix lying just above the upper part of the fetal skull (b). Rotation of the 2D transducer by 90° (c) provides a view of the cervix (d) [5]
50.5 Ultrasound Prior to Induction of Labor
Induction of labor is a common obstetric intervention, performed in about 20% of pregnancies [6]. However, about 20% of women having induction of labor, need a cesarean section for delivery either because of failed induction, failure to progress in labor, or fetal distress [7].
50.5.1 Preinduction Cervical Length
The measurement of cervical length within 24 h prior to the induction of labor by means of transvaginal ultrasound is an excellent predictor of the likelihood of vaginal delivery.
The Pandis et al [8] suggested 28 mm for the cervical length and 3 as the Bishop score are the best cutoff points for the prediction of successful induction. However, the cervical length appears to be a better predictor than the Bishop score, with a sensitivity of 87% and a specificity of 71% compared to 58% and 77%, respectively [8].
There will be reduction in the need of prostaglandin by 50% for the induction of labor in nullipara at term if 28 mm is the cut off used for pre-induction transvaginal cervical length [9].
Other parameters like angle of progression and elastography score of the cervix have also been used to predict their accuracy, but have not found to be useful [10].
Preinduction transvaginal cervical length of 28 mm or less measurement is a useful predictor for the likelihood of vaginal delivery.
50.6 Ultrasound-Based Prolonged Pregnancy Clinic
The risk of intrauterine and postnatal death is increased from 2.4 per 1000 pregnancies at 40 weeks to 5.8 at 43 weeks. Hence, postdate pregnancy is the most common cause of induction of labor.
Rao et al. [11] proposed an ultrasound-based assessment of the cervix and well-being of the fetus around the 40th week of gestation. Unless there is evidence of a specific medical or obstetric indication, induction of labor is delayed by 7–10 days, thereby increasing the percentage of females going into spontaneous labor by about 80% [ 11].
Hence, the concept of ultrasound-based prolonged pregnancy clinic has been introduced.
50.7 Ultrasound During Labor
Digital vaginal examination is used to assess the progression of labor. The parameters assessed are the cervical dilatation and effacement and head rotation and station. All these parameters can be assessed by means of ultrasound as well. Just by using a combination of transabdominal and transperineal ultrasound, we can gauge the progress of labor.
The major advantage is that the ultrasound being noninvasive and nonintrusive modality, avoids the discomfort associated with the digital vaginal examination. We are not proposing to do away with the digital vaginal examination but to use ultrasound as an adjunct to it. By using ultrasound, the number of per vaginum examination can be decreased, and the incidence of chorioamnionitis can be decreased from 10% for 13 vaginal examinations [12] to 4% for 2 DVE.
50.7.1 First Stage
The parameters assessed in the first stage are head rotation, head station, and cervical dilatation.
50.7.1.1 Head Rotation
Head rotation is the easiest parameter to assess on the scan. As a obstetrician I feel, it is the most difficult parameter to assess on DVE especially in the presence of caput. It has been seen that the assessment of the position of the occiput is inaccurate in majority of the DVE (digital vaginal examinations) [12, 13].
When we do DVE, we assess the head rotation by determining the position of the posterior fontanel and label it according to a 12-hour clock (with 12.00 h representing occiput anterior position and 03.00 h representing left occiput transverse position). Almost similar method was proposed by Akmal et al. [14].
Determination of occipital position
Cavum septum pellucidum is present toward left and occiput toward right (GE denotes right side of the probe). The midline echo is almost horizontal as shown by arrow, hence ROT
The interobserver agreement was nearly 15° in 90% of cases and 30° in all cases for sonographically determined fetal occiput during labor [14].
The use of vaginal examination, transabdominal ultrasound, and transvaginal ultrasound to determine fetal head position was compared by Zahalka et al. They found that the transvaginal ultrasound was the most precise method [15].
A high body mass index is the only significant factor that can make transabdominal ultrasound exploration difficult.
When we did a pilot study on about 12 women, we found that head rotation was easily picked up on scan by suprapubic transabdominal scan in 100% of examinations.
50.7.1.2 Head Station
Ultrasound during labor can clearly determine the position of the fetal head. This information provides objective data to pinpoint the labors whose progress is slow.
The reliability of DVE in the assessment of fetal head station was demonstrated by Dupuis et al. [17] using a birth simulator.
A fetal head mannequin was placed in the birth simulator and the operators then determined the head position clinically. The head position was incorrectly assessed by 50–88% of residents and in 36–80% of cases for obstetricians. The misdiagnosis of a station as midpelvic rather than high-pelvic accounted for 88% and 67% of the errors made by residents and obstetricians, respectively. This misinterpretation of the head station can have serious implications for the management of patients in labor.
