The last 3 decades have seen ultrasound evaluation become an integral part of obstetrics and gynecology. This diagnostic tool plays an essential role in the management of the pregnant woman during her entire pregnancy as well as on labor and delivery (L&D). However, while the ultrasound operative skill set continues to improve in most ultrasound units, the same cannot be said for most L&D floors. It is unfortunate that providers with greater ultrasound operating skills rarely participate in the care of patients outside the ultrasound unit, while the average “laborist” possesses limited sonography skills. In addition, it is the authors’ experience that the ultrasound machines used in most L&D units are usually old and often are rejected pieces from another service.

In the United States, most L&D units consists of a triage area (or obstetric emergency room), operating rooms, and delivery suites. The sonogram machines used at these units should ideally be portable and easily moved from patient to patient, with ultrasound knobology that is easily mastered by the average operator.

This chapter addresses how ultrasound can be utilized in the management of a patient that presents on L&D and how this management/diagnostic tool can be optimized in the care of the obstetric patient.

Ultrasound Equipment and Performing the Ultrasound

As aforementioned, this should be a portable machine, easily moveable from room to room or patient to patient. It is usually battery operated to enhance portability and rapidity of use. Curvilinear transabdominal probes at a frequency of 2 to 5 MHz and vaginal probes at a frequency of 5 to 10 MHz should be available. Departmental protocols should be created to maintain cleanliness and sterility of probes between uses. Documentation is essential for any ultrasound examination, whether in the ultrasound unit, the clinic, or on L&D. This is to allow later evaluation and follow-up, if necessary but is also imperative from a medico-legal standpoint. Unfortunately, in many L&D units, ultrasound is performed as a quick procedure with no archiving of images and no official record. Since clinical decisions are made, based upon this ultrasound, regulations and protocols must be in place for an official report, with clear permanently recorded images. as recommended by AIUM, for instance.¹ The images may be stored on film, digital format, or, ideally, in a Picture Archiving and Communication System (PACS). In most cases of ultrasound carried out on L&D, this will consist of a “limited scan.”¹ This is performed when a specific question requires investigation, such as confirmation of fetal heartbeat, fetal presentation, or placental location.² According to AIUM, in most cases, a limited ultrasound exam is appropriate only when a prior complete examination is on record. The appropriate CPT code for such an examination is 76815 (“Ultrasound, pregnant uterus, real time with image documentation, limited, eg, fetal heartbeat, placental location, fetal position and/or qualitative amniotic fluid volume, 1 or more fetuses”). If the study is more detailed, code 76805 (first time study) or 76816 (follow-up to reassess size and interval growth for instance) should be used. This code may be used once per fetus, hence billed twice for twins, three times for triplets, etc. If a biophysical profile is performed, code 76819 should be reported, once per fetus, with modifier 59 for each additional fetus.

Ultrasound applications in L&D can be classified into basic and advanced.³ These applications can be further classified into antepartum, intrapartum, and postpartum depending on the period of application.

Basic Application of Ultrasound in the Labor and Delivery Unit

Documentation of fetal presentation has been described as the commonest basic application of ultrasound in L&D.^3-6 This can be utilized in the antepartum period prior to admission for induction of labor, to create a plan of care for patients with prelabor rupture of membranes, and to assist in the performance of external cephalic version. It is also useful in the intrapartum period for plan of care in women in preterm labor, or the emergent care of women unknown to the unit in whom imminent delivery is suspected. In addition, fetal presentation is critical in the intrapartum care of multiple gestations, especially in the delivery of the second twin.

Management of various antepartum complications is largely dependent on the estimated gestational age of the fetus. For example, preeclampsia with severe features may be managed conservatively if the pregnancy is estimated to be less than 34 weeks gestational age, but delivery is almost always indicated if the pregnancy is deemed 34 or more weeks. Determination of the fetal gestational age utilizing fetal biometry parameters such as biparietal diameter, head circumference, abdominal circumference, and femur length is a skill that the average L&D practitioner should have acquired during residency training in the United States. Estimation of fetal weight is usually routine prior to induction of labor as well as in women in the early phase of labor. It is mostly performed to determine the presence of fetal macrosomia, which in turn may avert maternal morbidity. Unfortunately, ultrasound has been demonstrated to be unreliable in predicting macrosomia.^7,8 Furthermore, multiparous women have been observed to provide a more accurate self-estimation of fetal weight compared to their providers who estimated fetal weight clinically or by ultrasound.⁹ It should be noted that inaccurate prediction of fetal macrosomia predisposes to increased labor abnormality diagnosis and in turn cesarean section deliveries.¹⁰

Fetal biophysical profile constitutes almost 10% of ultrasound evaluations in L&D units in the US.¹¹ This is usually performed due to complaints of reduced or absent fetal movements and as part of evaluation in pregnancies thought to possibly being complicated by placental insufficiency and its consequence, fetal growth restriction.

Localization of the placenta prior to cesarean section delivery by ultrasound and avoidance of its transection if located anteriorly has been described to significantly reduce blood loss,¹² especially in pregnancies complicated by anteriorly located placenta previa.¹³ Furthermore, ultrasound evaluation prior to surgery may aid the surgeon in planning for alternate uterine incisions such as paramedian classical uterine incision¹⁴ or transverse fundal uterine incision,¹² which have both been demonstrated to significantly reduce surgical blood loss. Transvaginal ultrasound examination is an invaluable tool in evaluating the antepartum patient unknown to the service who presents with painless antepartum bleeding to confirm or rule out placenta previa (Figures 16-1A and B).

Obesity complicates over 30% of all pregnancies in the US.¹⁵ Obesity, in addition to uterine fibroids, peculiar maternal body habitus, surgical scars, and fetal malpresentation, may attenuate our ability to achieve and or maintain continuous electronic fetal monitoring in the laboring patient. Ultrasound can be used in the aforementioned clinical scenarios to locate fetal heart tones and achieve optimal placement of the cardiotocography transducer. It can also be used to emergently confirm fetal bradycardia during labor, especially when there is doubt as to whether the cardiotocography transducer is interpreting maternal vessel movement as fetal heart rate.

Depending on the skill set of the clinician on the labor floor, cervical length evaluation of the patient with preterm uterine contractions (Figure 16-2A) or evaluation of the relationship of a lower segment uterine myoma or pelvic mass with the internal cervical os (Figure 16-2B) may be regarded as basic or advanced utilization of ultrasound. Examination of the cervix when uterine contractions are present may help differentiate between “false” versus “real” labor.¹⁶ Patients in true labor were found to have shorter cervical length with a median of 1.3 cm (range 0.5-4.1) versus 2.4 cm (range 1.0-5.0; p < 0.001) in patients who did not continue to deliver.¹⁶ Furthermore, there are reports of intrapartum diagnosis of potential life threatening conditions such as uterine rupture and vasa previa with the aid of ultrasound.^17-19

Advance Application of Ultrasound in the Labor and Delivery Unit

Operative vaginal delivery is beneficial as cesarean section delivery, and its associated morbidities can be avoided.²⁰ Unfortunately, the last 2 decades have seen a decline in the rate of operative vaginal delivery from 9% to 3% in the United States.²¹ Accurate determination of the fetal head position is critical to the safe performance of an instrumental delivery.²⁰ Fetal head position is commonly evaluated by digital vaginal examination. However, there is ample evidence to suggest that this method of evaluation may be inaccurate,^22-27 with a particular study reporting an inability to define fetal head position in over 30% of cases.²⁴ These inaccuracies are not limited to the stage of labor, as discrepancies between digital- and transabdominal ultrasound-obtained fetal head position of at least 45 degrees in 53% and 39% in the first and second stages of labor, respectively, have been demonstrated.^22,23 In another study, fetal head position could not be determined in 61% and 31% in the first and second stages of labor, respectively.²⁶ It is concerning that in over a third of women in the second stage of labor, the fetal head position was either inaccurately determined or could not be determined with digital vaginal examination. Hence, operative vaginal delivery would not be performed safely if indicated in either of the aforementioned clinical situations. It should also be noted that other investigators have demonstrated transvaginal sonography to be superior to both transabdominal ultrasound and digital vaginal examination in determining fetal head position.²⁸ Therefore, it is safe to state that intrapartum ultrasound may assist in the correct placement of the vacuum cup or forceps blades, with the potential of improving the success rate of operative vaginal delivery,^24,25,29 in addition to its safety (Figure 16-3).

Determining fetal station and, in essence, labor advancement with digital physical examination has also been demonstrated to be inaccurate in most instances. Using a mannequin equipped with miniaturized sensors, specialist obstetricians and trainees misdiagnosed the fetal station 67% and 88% of the time, respectively.³⁰ The implications of such misdiagnosis cannot be overemphasized. Prediction of delivery mode and fetal head progression in labor with aid of intrapartum sonography has been demonstrated with transabdominal and transperineal/translabial ultrasound. These studies have used parameters such as angle of progression of the fetal head,^4,31,32 head direction,^33,34 and progression distance^35,36 to predict progress of labor and or probability of successful vaginal delivery. The angle of progression of fetal head is described as the angle between a line drawn through the midline of the symphysis pubis and a line from the inferior apex of the symphysis to leading part of the fetal skull (Figure 16-4A). An angle of at least 120 degrees or greater has been shown to almost always be associated subsequent vaginal delivery whether spontaneous or assisted.^4,31,32 Head direction has been defined as the angle between the infrapubic line of the pelvis (a line perpendicular to the longer diameter of the pubis starting from the inferior border) and another line drawn perpendicular to the widest diameter of the fetal head (Figure 16-4B).³⁴ Utilizing this ultrasound technique, the direction of the head is determined to be either head down, horizontal, or head up. The best direction for the fetus to have is head up, which implies that the line perpendicular to the widest diameter of the fetal head points ventrally, which is usually at an angle of 30 degrees or greater. This is a favorable prognostic sign for vaginal birth. An angle of less than 0 degrees is regarded as head down, while an angel between 0 degrees and less than 30 degrees is regarded as horizontal. Progression distance has been defined as the minimum distance between a line through the inferioposterior margin of the pubis symphysis and the border of the fetal skull cap (Figure 16-4C).³⁵ Some experts regard this as the most important measurement as it correlates well with fetal station, which is necessary prior to the attempt of any form of instrumental vaginal delivery. Among these three methods for measuring fetal head progression in labor, angle of head progression was reported to have the best intra- and interobserver reproducibility when compared to the other 2 described methods.³⁶ Some ultrasound companies include softwares to follow progression of labor and descent of the presenting part.