Probably one of the commonest uses of portable ultrasound in birthing units has been to assess presentation. The position of the head and orientation of the longitudinal axis of the spine should be used.
On occasion, the suspicion of a cord presentation will arise either from a vaginal examination or from marked decelerations related to contractions. Transperineal or transvaginal ultrasound, aided by color Doppler, can assist diagnosis (Fig. 37-3).
Cord presentation seen on color Doppler views. Pulsed Doppler can confirm fetal umbilical arterial wave pattern.
Abdominal delivery is the only option for an abnormal heart rate. If there is no suspicion of fetal compromise, expectant management can be undertaken provided there is rapid access to emergency Cesarean section because there is an ever-present risk of acute cord prolapse. Cases with intrapartum resolution of the cord presentation as labor progresses have been reported. It is also important to distinguish cord presentation from vasa previa. The latter may be associated with an accessory placental lobe and will usually only have two vessels. Alternatively, there may be a membranous cord insertion into the lateral margin of a low placenta (velamentous cord insertion). In such case, vaginal delivery is not recommended.
Transabdominal ultrasound can usually exclude previa, but a transvaginal or transperineal approach is required to diagnose it (see Chapter 31). Clinicians should avoid the temptation to search for abruption that may delay proper management because the diagnostic sensitivity of ultrasound is poor. However, moderate and large hematomas can be seen by ultrasound (Fig. 37-1).
There has been renewed interest in vaginal breech delivery. Most guidelines recommend knowledge of the type of breech presentation to identify the footling breech at risk of cord complications (Fig. 37-4).
Translabial ultrasound in sagittal views showing footling breech presentation in a woman presenting in preterm labor.
The longitudinal orientation of the cervical spine should be visualized to exclude the hyperextended “star-gazing” position associated with an increased risk of neurologic complications. Finally, attempts at vaginal birth are often tied to certain birthweight ranges. If the breech was previously undiagnosed, some attempt at assessing this using biometry will be necessary.
Occipitoposterior (OP) positions remain a prominent cause of dystocia in labor and operative delivery. Transabdominal ultrasound can reliably assess head position using the position of the fetal eyes and the spine in a cross section through the chest (Figs. 37-5A, B). Translabial ultrasound can also be used (Fig. 37-5C).
A. Transabdominal views showing fetal eyes looking straight upward in a direct occipitoposterior (OP) position. B. The spine can help when maternal acoustic shadows don’t permit visualization of fetal eyes as in this occipitoanterior position. C. Transverse views of translabial scan showing the midline falx and the choroid plexi diverting away from the midline toward the back of the occiput, which here shows an OP position. D. Asynclitism can be demonstrated using a translabial view in transverse section. E. The interrupted red line overlying the falx divides the brain into two equal halves with the yellow lines defining the angle between the falx and the midline pelvis (known as midline angle and can be used as an indicator of birth progress).
It has shown that transient OP positions are seen in up to one-third of labors but that the majority rotate to occipitoanterior (OA) even after full dilatation. Epidural anesthesia has been shown to reduce this rotation, leading to a higher persistence of posterior positions. Most importantly, ultrasound has shown how inaccurate digital vaginal examination is at assessing head position, with some studies reporting error rates as high as 75 percent. The accuracy of digital assessment was poorest when the head was in a non-OA position or when the station was not below the ischial spines. These are the exact clinical situations that can be associated with difficult instrumental deliveries. Correct placement of the ventouse cup or forcep blades is predicated by an accurate assessment of fetal position.
Another factor to be considered when assessing the head position is synclitism (Figs. 37-5D, E). Asynclitism can complicate operative vaginal deliveries. This can be achieved by ensuring the brain midline (falx) is seen.
Several methods have been studied using transperineal ultrasound with either direct measurements using the symphysis pubis as a landmark (angle of progression, progression distance, and head symphysis distance) or indirect method using the fetal head as a landmark (head perineal distance) (Figs. 37-6A, B).
In a sagittal view of translabial scan, the symphysis pubis (*) is used as the landmark for head station. The yellow “infrapubic line” drops at a 90° angle to the front of the symphysis pubis and represents the “–3” station. Ischial spines (representing “0” station) are therefore 3-cm anterior to the “infrapubic line” (x). Angle of progression (AoP) is represented by the red lines. Molding (green) and caput succedaneum (blue) are seen.
Angle of progression (AoP) is the most studied method with the strongest evidence. It is the angle between the long axis of the pubic bone and a line from the lowest edge of the pubis drawn tangentially to the deepest bony part of the fetal skull. Vaginal delivery was 88 percent with AoP ≥110 and 38 percent with AoP <110 in a prolonged first stage of labor. Tutschek et al have developed a table that correlates the AoP and the head station.
Other measurements include: progression distance (PD), head symphysis distance (HSD), and head perineum distance (HPD). References at the end of this chapter provide more details about each technique.
The angle between the longest recognizable axis of the fetal head and the long axis of the pubic symphysis, measured in a midsagittal transperineal view.
Utilizing the angle of head rotation as an indicator of birth progress. This can be assessed thorough the same view in Fig. 37-5E.
After delivery of the first twin, the lie of the remaining twin needs to be assessed along with the FHR. These can both be accomplished quickly and efficiently by real-time scanning. Knowing the fetal cardiac position allows prompt and accurate placement of the external monitors’ Doppler probe. Alternatively, the heart rate can be monitored by ultrasound, because the relatively rapid descent that will occur will require frequent readjustments of the Doppler probe. If either a breech extraction or internal podalic version is required, the position of the fetal feet can be identified before the procedure is started. Real-time guidance can be provided to the accoucheur during intrauterine manipulation.